Earthquakes and other warning signs began 30 years ago this week in the Cascade volcano
By Summit Voice
~Source: U.S. Geological Survey
SUMMIT COUNTY — It was thirty years ago this week that scientists in in the Cascades started tracking a swarm of earthquakes that would ultimately culminate in the May 18 eruption of Mount St. Helens. To mark the anniversary, the U.S. Geological Survey has published a special web site.
In the one-week span between March 15 and March 21, more than 100 quakes were recorded. By comparison, there were only 44 quakes in the preceding five years. A 4.2 earthquake on March 20, 1980 gave geologists the first real warning that the mighty volcano might be preparing to blow its top for the first time since 1857. The strong temblor on March 20 triggered massive avalanches on the mountain’s snowy flanks.
The March 20 quake was unlike any other that had previously detected in the area, pinpointed near the summit and at a very shallow depth. Seismologists suspected the earthquakes but didn’t say so publicly. They did warn the U.S. Forest Service that more earthquakes might trigger more snowslides, endangering climbers and skiers enjoying prime spring conditions on the mountain. Subsequently, the Forest Service closed the mountain above timberline.
During the following weak, quakes increased and scientists became convinced that an eruption was building. On March 27, rising magma made contact with water, resulting in what geologists call phreatic eruptions that would continue until just before the cataclysmic explosion of May 18.
U.S. Geological Survey officials issued a hazards watch on March 27, as observers flying over the volcano reported seeing a hole in the icecap with a gray streak on the snow. The same day, people near the volcano reported a loud boom. A 200-foot crater formed at the summit of the mountain and a series of cracks criss-crossed the area, leading to concerns about the potential for large rock slides. Hundreds of people were evacuated from the area.
Scientists flying over the mountain detected sulfur-dioxide, a sign that gases were being released by a high-temperature magmatic source in Mount. St. Helens. Soon after, seismologists reported the first harmonic tremor — lyrical and ominous-sounding at the same time. The long-duration release of seismic energy was sign that magma was on the move.
By the end of March, the U.S. Geological Survey reported that a carnival-like atmosphere prevailed around the base of the mountain, with numerous planes flying around the summit and hundreds of sightseers parking along roads in the area to get a view of the action.
Local county officials declared a state of emergency to manage closures, and officials fielded calls from gamblers requesting the number of eruptions in the previous 24 hours , and from people claiming the mountain’s restlessness was due to the desecration of Native American graves in the area. Plumes of smoke from the explosions climbed to an altitude of 20,000 feet, with ash-fall reported in Portland and Vancouver.
Tension developed between emergency managers intent on keeping people safe, and logging companies pushing for more access to restricted areas. By early April, the mountain was bulging by as much as 300 feet, with the deformation continuing right up through May 18. Glaciers on the mountain started to melt and slip into rivers draining the area, leading to concerns about downstream flooding. Forest Service and local budgets were stretched by the need to maintain access restrictions.
Through the middle of April, the mountain appeared to quiet down, at least superficially, with fewer explosions. But earthquakes continued, and scientists bustled around the area to set up monitoring devices and cameras to record what would become the most-studied volcanic eruption in history.
In late April, eruptions slowed, then stopped. Officials said that, because of the lull in visible activity, they were having a hard time convincing people of the growing hazards. But for a period in early May, the bulge on the north side of the peak grew at an astonishing rate of five feet per day. The quiet period ended May 7 when steam and ash explosions resumed.
The growth of the bulge was documented by scientists in a sequence of photographs showing how the mountain was visibly changing.
In mid-May, just a few days before the eruption, property owner in the Red Zone demanded access to their cabins and threatened to overwhelm guards at the roadblocks. Tourists from around the world had gathered near the volcano to watch geology in action. May 17, just 24 hours before the ultimate explosion, the government relented and permitted a convoy of cars into the area to retrieve personal possessions.
The next day, 57 people — including scientists, reporters, loggers and campers — died when the mountain exploded. Only four of them, including USGS researcher Dave Johnston and stubborn Spirit Lake Lodge owner harry Truman, were inside the restricted area. The other two victims in the Red Zone were amateur vulcanologists Bob Kaseweter and Beverly Wetherald who had permission to take readings at Spirit Lake. The other victims, some as far as 13 miles from the mountain, were considered to be in safe areas.
Check out the excellent USGS web presentation on the 1980 Mount St. Helens eruption here. The site includes a day-by-day scientific diary starting with the first quakes in mid-March and ending the day of the eruption.