Mount St. Helens Precursory Activity

Aerial view of crater taken on April 12, 1980. Highly fractured area to the right of the crater was being pushed outward. USGS photo courtesy of Dan Miller.

Holding Pattern - Seismic activity tapered off and even diminished during several days. Explosions continued, but decreased in size and frequency. USGS scientists became increasingly concerned over the growing instability of glaciers and rock on the north flank of the volcano. A comparison of aerial photographs taken in 1979 and April 1980 confirmed parts of a "bulge" one mile long and one-half mile wide on the north flank had moved outward by as much as 320 feet. Sightseers and media continued to press the closure boundaries established by authorities. Mounting expenses for both volcano monitoring and maintaining roadblocks caused several agencies to scale back operations or pull out altogether.

Monitoring Deformation

Aerial photos taken on April 12 allowed scientists to construct a topographic map of the volcano. When this map was compared to a similar map made from aerial photos taken in August 1979, it showed that parts of the mountain had shifted substantially. USGS scientists began to monitor the volcano more closely for signs of deformation . Electronic distance measurement (EDM) units (shown at right) were used to measure distance from fixed base stations to reflectors (inset) placed at various points on the volcano. Theodolites were used to measure horizontal angles between the two. By making daily or near daily measurements with both types of instruments scientists were able to document which parts of the mountain were moving and how quickly.

Map showing location of base stations (black) and target reflectors (red) for EDM surveys of Mount St. Helens between April 25 and May 17, 1980. Figure modified from USGS Professional Paper 1250, p. 147.

Mount St. Helens from Spirit Lake, April 11. The two prominent rock outcrops halfway up the slope are Dogs Head (left) and Goat Rocks dome (right). The area to the right of Dogs Head, including Goat Rocks dome, was being pushed outward. Survey targets placed at key locations provided USGS scientists with information about the growth of the bulge. By late April the increasing steepness and instabiligy of the north flank prompted concerns over large rock avalanches that could reach Spirit Lake. Note the avalanche to the lower right of Dogs Head (dark streaks). USGS photo courtesy of Don Swanson.

Other techniques were also employed by USGS scientists to monitor deformation of the volcano. Tiltmeters were deployed at several locations around the volcano to detect swelling or subsidence of the cone. Dry tilt measurements were made at the Timberline Viewpoint using more traditional surveying techniques. The photos below show USGS scientists performing dry tilt measurements at Timberline Viewpoint in mid-April.

Daily Log

April 12 - Instruments detected ground swelling near Goat Rocks. Scientists who worked in the area reported seeing new ground cracks. The progressive cracking and shifting of the ice on the north side below the crater caused increased concern that a large mass of ice and rock could avalanche into the North Fork Toutle River. A 27-minute episode of harmonic tremor recorded in the afternoon ended with a magnitude 4.5 earthquake. This would be the last episode of harmonic tremor recorded for nearly one month. Aerial photos were taken to compare with previous photos to document changes of the volcano. William Menard, director of the USGS, arrived in Vancouver to observe the volcano and inspect the instrument and data collection activities. Members of the Lower Columbia Amateur Radio Association set up stations on the Toutle River, the Yale School, state department of emergency services headquarters in Olympia, and at the emergency services offices in Kelso as the main line of communications for state emergency services.

April 14 - Cloudy weather prevented observations. No new ashfall was reported. Seismometers show continued sizeable earthquakes, but no explosions. The Timberline tiltmeter showed continued slight subsidence of the central part of the cone.

EDM target (orange, center of photo) on Goat Rocks dome. Note highly fractured ground surface near target. USGS photo courtesy of Dan Miller.

USGS scientist uses a geodimeter to electronically measure distances between the Smith Creek Butte instrument site and targets on the volcano. USGS photo courtesy of Pete Lipman.

USGS scientists install tiltmeter near Mount St. Helens. USGS photo courtesy of Pete Lipman.

April 15 - Only one explosion was documented. The volcano became visible by midday. No changes in ground tilt were recorded. Washington Department of Fish and Game announced that Merrill Lake, Spirit Lake and Swift Reservoir would remain closed to fishing to reduce the temptation of the public to evade roadblocks. The FAA began investigating 7 aircraft that violated restricted airspace around Mount St. Helens. Pilots faced losing their license and fines up to $1000. Skamania County announced the removal of their manned roadblocks on the Lewis River Road and N73. According to a Daily News article, the county cited expense, public harrassment, the stable pattern of explosions at the volcano, and the impracticality of keeping people out of the network of logging roads as snow melts. Many National Guard troops have also left. Ten continue to man the roadblock on State Route 504.

Close up of phreatic explosion. USGS photo courtesy of Richard Waitt.

April 16 - Nine small steam explosions and some ashfall reported during the day. Seismic activity reached the lowest point for April, with 30 earthquakes of magnitude 3.0 or larger and 3 of magnitude 4.0 or larger. Tiltmeters continue to show no changes. A project to collect ash samples (started on April 14) in snow at 30 stations was completed. Burlington Northern asked the USFS to administer their property on the summit as closed. Television crews who had landed there days before were immune to prosecution because they landed on property that was outside of the closure enforced by the USFS.

USGS scientists use a correlation spectrometer (COSPEC) to measure sulfur dioxide at Timberline. USGS photo courtesy of Tom Casadevall.

April 17 - Twelve small plumes erupted from the crater between 7:00 AM and 1:30 PM. Earthquake activity remained low even though there were two of magnitude 4.6 recorded. Scientists used the calm to conduct gravity surveys around the mountain, place surveying targets near the summit, and to estimate the total amount of ash accumulated. Due to continued ground vibrations from earthquakes, fracturing of ice and rocks, subsidence of the summit graben, and accelerated melting of snow and ice, scientists concluded that there was an increased possibility of a large avalanche on the north flank.

COSPEC at Timberline. Note the bulging area on the north flank between the two dark masses of rock (Dogs Head and Goat Rocks dome). USGS photo.

April 18 - Steam plumes only rose to about 500 feet above the crater and seismicity remained moderate. As the volcano settled into a "holding pattern" the USGS went from daily information releases to twice weekly. The roadblock on State Route 504 was moved to milepost 19 near the State DOT Kid Valley Maintenance Shed and remained staffed by the Washington State Patrol.

April 19 - Local scientists familiar with the volcano recognized that the upper north flank of the volcano was moving outward, but disagreed on why. Some thought the summit graben subsidence was to blame, others magmatic inflation pushing from within the volcano. There were 37 earthquakes larger than magnitude 3.0 recorded, including 8 larger than magnitude 4.0. No harmonic tremor has been recorded during the last week. US Soil Conservation Service snow samplers found that snow melted faster where covered with a thin layer of ash. USGS hydrologists have not documented any unusual changes in stream levels.

April 20 - No explosions occurred, but seismicity reached the highest point in two weeks. The record showed 47 earthquakes larger than magnitude 3.0, including 7 larger than magnitude 4.0. Fising season opened, doubtlessly contributing to the number of sightseers who jammed roads leading to the mountain.

Crater continued to sink as the north flank moved outward. USGS photo.

April 21 - Poor visibility prevented direct observations of the volcano. The lull continued, with seismicity returning to levels similar to those of the previous week.

April 22 - Improved weather made aerial observations possible. New snow was dusted with ash in only one place. Small bursts of steam around noon were the only activity observed. Other steam vents were visible on the eastern part of the summit area. Tiltmeters continued to record changes that were inconclusive. Forty two earthquakes larger than magnitude 3.0, including 5 larger than magnitude 4.0, were recorded. In a Columbian article, USGS scientist Dwight Crandell warned those using cabins in the Northwoods area south of the volcano to have an evacuation plan and route ready.

April 23 - No explosions were large enough to add to the light coating of ash seen on fresh snow on the 22nd. Two new steam vents were observed in the crater. There were 38 earthqaukes larger than magnitude 3.0 recorded, including 11 larger than magnitude 4.0. Tiltmeters at Timberline and Spirit Lake showed a slight tilting downward to the northwest. Tilt on the south side was downward to the southwest. The overall pattern suggested the volcano might be inflating due to increased magma pressure from below. Gas samples were collected from the steaming areas near the summit. Analyzed later, they would be found to contain high concentrations of sulfur dioxide. Data became available from a comparison of aerial photographs taken April 12 with those taken in 1979. From the comparison it appeared some places on the volcano 250 feet higher than 1979. Due to continued quiet, USFS officials scheduled discussions on opening some areas around the volcano to restricted use. USGS scientist Dwight Crandell warned, however, that avalanches from the unstable bulge could cause floods by suddenly raising the level of Spirit Lake or temporarily damming the North Fork Toutle River. In response to demands for more frequent information, USGS scientists resumed daily information releases.

Scientist changes the magnetic tape of a portable data recorder at one of the seismic stations at Mount St. Helens. USGS photo courtesy of Elliot Endo.

April 24 - Fumaroles continued to steam near the summit crater, but no explosions were large enough to throw material outside of the crater walls. The number of earthquakes recorded has been about the same as in previous days, but average magnitudes are higher. There were 9 earthquakes larger than magnitude 4.0, including one of magnitude 4.7. A total of 38 earthquakes larger than magnitude 3.0 were recorded. USGS Volcano Hazards Project staff released a map showing the possible dangers presented by the bulge area and emphasized the potential for avalanches. The Longview Daily News quoted Dave Johnston on the bulge, " It would seem to indicate something is pushing up from beneath. The obvious speculation is that it might be magma. A major deformation like this is pretty extreme." In the same article they reported that the Red Cross took national disaster workers off standby alert and asked Cowlitz County to return all extra shelter equipment borrowed from division headquarters in Portland.

April 25 - Fumaroles continued to steam and seismic activity continued at about the same level as in previous days. There have been approximately 1560 earthquakes larger than magnitude 3.0, including more than 200 larger than magnitude 4.0, recorded since March 20. USGS scientitsts conducted an EDM survey to 5 targets on the upper slope of the volcano as well as two additional reference points. Additional survey targets were installed to help define the size, shape and growth of the bulge. At the daily press briefing USGS scientist Dwight Crandell described the Forsythe Glacier as "humped up into one big black rock mass" 300 feet higher than before March 27. A large earthquake or steam blast could send the 150 foot thick mass of ice rushing down the mountain at speeds up to 180 miles per hour. At that speed it would reach Spirit Lake in two minutes. "There will be no way to warn people. If they are in the path, they won't be able to get out of the way." The Spirit Lake highway (State Route 504) would be blocked by the avalanche debris.

Smith Creek Butte EDM site, April 25, 1980. USGS photo courtesy of Pete Lipman.

USGS scientists use a geodimeter to perform electronic distance measurements (EDM) from the Toutle instrument station. USGS photo courtesy of Pete Lipman.