VOLCANO MADNESS: [Geology2] Earthquakes, Mount St. Helens, and the May 18, 1980 Eruption

Earthquakes, Mount St. Helens, and the May 18, 1980 Eruption

March 22, 2010 by Arne

It's easy to forget, or simply never realize, that the big St. Helens eruption in 1980 was triggered by an earthquake. A USGS feature on St. Helens explains:

May 18, a Sunday, dawned bright and clear. At 7 a.m. Pacific Daylight Time (PDT), USGS volcanologist David A. Johnston, who had Saturday-night duty at an observation post about 6 miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. Even considering these measurements, the status of Mount St. Helens' activity that day showed no change from the pattern of the preceding month. Volcano-monitoring data-seismic, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature-revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later.

About 20 seconds after 8:32 a.m. PDT, apparently in response to a magnitude-5.1 earthquake about 1 mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston.

Geologists Keith and Dorothy Stoffel, who were in a small plane over the volcano's summit when the eruption started, wrote that they "noticed landsliding of rock and ice debris in-ward into the crater… the south-facing wall of the north side of the main crater was especially active. Within a matter of seconds, perhaps 15 seconds, the whole north side of the summit crater began to move instantaneously. … The nature of movement was eerie…. The entire mass began to ripple and churn up, without moving laterally. Then the entire north side of the summit began sliding to the north along a deep-seated slide plane. I (Keith Stoffel) was amazed and excited with the realization that we were watching this landslide of unbelievable proportions. … We took pictures of this slide sequence occurring, but before we could snap off more than a few pictures, a huge explosion blasted out of the detachment plane. We neither felt nor heard a thing, even though we were just east of the summit at this time."

In the Seattle Post-Intelligencer of Tuesday, April 21, 1987, Debera Carlton wrote that Jim Zollweg of the U.S. Geological Survey at the UW and Chris Jonientz-Trisler of the UW seismology lab had developed a new understanding of the eruption. Carlton explained:

The two seismologists have been able to reconstruct the 1980 eruption and reinterpret what happened inside the mountain in the months before the big explosion. They've discovered that "subedificial" quakes played a key role in what happened at Mount St. Helens. At the time of the eruption, scientists had recorded these small quakes but did not attach any significance to them.

Subedificial means "beneath the edifice," which in this case was the volcano.

"Right at the surface is where the volcano erupts, so scientists at the time were paying a lot of attention to that," Zollweg said. "That's where the mountain was actually bulging, and that's where the mountain failed. A magnitude 5.1 earthquake triggered the landslide that essentially opened up the crater and released the magma and gases.

"But if they'd recognized these other types of earthquakes , which we have dubbed subedificial quakes, they would have known that there was also stress being applied deeper within the system, not just at the top where the bulge was," he said. "There was magma moving down at depth, which is one thing they didn't realize at all at the time.

"What we did was change the whole concept of what the volcanic system probably looks like."

Zollweg and Jonientz-Trisler decided to reanalyze the 1980 data after noticing subedificial earthquakes during an explosive eruption of St . Helens in March 1982. The seismologists wanted to know if these quakes had occurred in 1980 but might have been overlooked.

"This was the first time something like this had been done for a North American volcano," Zollweg said.
The two scientists analyzed an overwhelming volume of data from quake activity that occurred at Mount St. Helens from the time of the first steam eruption on March 27, 1980, to the explosion May 18.

More than 20,000 large earthquakes had been recorded in what the seismologists term the "shallow system," roughly occurring from one mile below sea level up into the cone, or the mountain itself. . . . The subedificial earthquakes lined up in a ribbonlike line directly below the bulge where the northern portion of Mount St. Helens eventually gave way.

A similar line of subedificial quakes was found on the west side of the mountain, indicating a second conduit system. This system, Zollweg said, begins deeper but does not extend as high within the volcano.

The seismologists not only located the conduit systems, but determined their approximate size.

"If you had X-ray vision and could look inside the mountain, you'd see a nearly vertical structure – a big pipe," Zollweg said. "It's elongated like a ribbon, and runs off at a slant. The width has to be very small. It can't be more than a few tens of yards across.

"The thinner the conduit, the less magma it can hold. The more magma that tries to pump through there, the greater the rate of rock breakage.

"We've described some very, very small pipes through which the magma travels, and the earthquakes are the result of this excess pressure, like when you try to push too much water through a pipe. If extra pressure is exerted, the pipe will really bulge. And since the pipe can't really bulge in this case, the only thing it can do is break the rock around it."

Eventually, the two seismologists came up with this play-by-play of what happened:

On March 27, 1980, the first steam eruption occurs at St. Helens. Subedificial earthquakes indicate magma moving through the conduit system.

Earthquake activity accelerates between April 5 and April 27-28, the time of "maximum pre-eruptive stress" around the conduits.

Earthquake activity decreases between April 30 and May 18.

On May 18, a magnitude 5.1 earthquake causes a huge landslide that rips away the mountain's bulge. This has the same effect as popping a cork off a champagne bottle. All the pent-up gases and magma explode out of the mountain.

"On May 18, the magma and gases were not ready to explode by themselves," said Jonientz-Trisler. "But after the landslide, the magma and gases were no longer contained and under pressure. They were then able to move up in the system."

The number and size of subedificial earthquakes were greatest in the eight hours after the May 18 eruption.

"After the cap came off, the subedificial activity was the most intense," Zollweg said. "When you take the cork off, anything down there can start coming up, so the magma was actually moving faster through the conduits after the eruption than before."

Finally, a Seattle Post-Intelligencer article by Mike Barber on Thursday, May 11, 2000, talked about Harry Truman, the old man of Spirit Lake. It said:

Just before St. Helens came to life in March 1980, Truman had been slowed by a series of mishaps and seemed to be deteriorating, said George Barker, who was the Skamania County Sheriff's resident deputy at Spirit Lake.

"That winter I was more concerned about an elderly guy staying warm and having food; he was getting older and getting tired," Barker said.

When the mountain started acting up, Truman seemed to come alive as well – especially when reporters started coming in on the helicopters that would land near his lodge.

"When they began coming around, he got another shot in life," Barker said. "He enjoyed the attention."

Rosen says Truman's unwillingness to leave the mountain had more to do with protecting his property than making a statement. Others say the headlines contributed to his refusal to come off the mountain – he felt obliged to live up to his press.

"I think he kind of got himself talked into a Catch-22 situation to stay," Barker said. "He wanted to come down. He was very much afraid of earthquakes.

"He felt, like everyone else, that he would be able to see lava start to ooze down and a news helicopter would come in and scoop him up at the last minute."

Nature had other ideas. The searing blast came at 200 mph.

"One scientist told us Truman probably had time to maybe turn his head," Rosen said.

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