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New earthquakes are disturbing central Oregon

Three weeks ago earthquakes began near South Sister Mountain in central Oregon, drawing the intense interest of Scott Burns, Portland State professor of geology, and geologists everywhere.

These weren’t huge shudders like the earthquakes that preceded the eruption of Mount St. Helens. They were more like little squiggles, causing disruptions in the molten rock which has been thrusting up slowly into a growing bulge in the ground near the South Sister Mountain.

But from little earthquakes do big earthquakes grow. If this one eventually turns into a big blow, it would not threaten any nearby towns; the Three Sisters chain is a wilderness area. But it would rain unwelcome volcanic ash onto skiers at Mount Bachelor to the south and almost certainly would unpleasantly affect the nearby city of Bend.

Actually, Burns said, he and other geologists have been surprised that these earthquakes were so long in coming.


“In the last five years, rising magma had created a bulge in the ground that was going up one inch per year,” he said. “Normally, when magma comes in, there are small earthquakes, but here there had been no earthquakes. Then three weeks ago, we had a swarm of earthquakes, proving that the magma is moving up.”

The growing bulge was detected by satellite measurements over the area for the past five years, but until three weeks ago there was no discernible evidence that anything of significance was happening underground.

“Will it erupt in our lifetime?” Burns pondered. “It may or it may not.” If it does erupt, the type of eruption may be more or less severe.

“The question is, will it produce andesite, which tends to be a more violent eruption,” he said. The Northwest crust has been subject to numerous flaws in the last 2,000 years but most of the volcanic activity has spewed out basalt, which tends to produce less violent eruptions.

“What type of magma is it? Basalt or andesite?” Right now, that is unknown.

Students in this area enjoy great opportunities to study the earth’s crust.

“We live in a dynamic environment,” Burns said, “We have great geology. Things are always happening. There is always something to talk about.

“Students here don’t have to read about it in textbooks. They can go out and see it. The next things that are happening in geology, we’re getting here in the Pacific Northwest.”

New technology has been a great boost to the study of geologic changes. The satellites that hover over under auspices of the U.S. Geological Survey have been able to measure these gradual increases in the bump near the South Sister, even though it has humped up only five inches in five years.

South Sister isn’t the only volcanic activity attracting attention these days. Mammoth Mountain in California has periodic earthquakes and has been exuding gasses so toxic, it is killing whole forests of trees due to an overage of carbon dioxide. Yellowstone National Park, Burns said, is actually one big volcanic caldera.

History shows the area has one tremendous eruption very 600,000 years and it has been 600,000 years since the last one, so a big one may be imminent.

“Are we due?” Burns speculated. “There are some indications it may come soon.”

Actually, there are presently active volcanoes in the ocean only 200 miles directly west of the Oregon coast. The University of Washington has received a grant to put exploratory devices and sensors on the Juan de Fuca ridge on the ocean floor to measure seismic activity there. This was an activity not even known until recent years, since the undersea eruptions did not shoot up out of the ocean surface.

Burns recently completed a term as national president of the Association of Engineering Geologists. He went on a national tour of all the association’s chapters, where he delivered presentations on one of his geologic specialties, site evaluation in terms of water analysis. Burns can claim to be an expert on this.

“I talked about lessons I have learned from 25 years of landslides due to weak slopes and weak soil,” he said. It’s rain that he attributes the damage to and he anticipates even more.

“As the population is increasing, we are going to be seeing more and more landslide disasters,” he predicted. He pointed to a slide at Kelso, Wash. in 1998 that took down 60 houses. He also recalled a major landslide at The Capes in Tillamook County is 1997.

He issued a word of caution, “If the ground has moved once, it has a very good chance of moving again.” There are places in the Portland area, particularly in Clackamas County, he said, where he would never build a house. He recalled that in 1996, when the area had eight or nine inches of rain in four days, landslides occurred at numerous locations as old landslides became reactivated.

“The problem is, there is no landslide insurance,” Burns said. “If the house moves down the hill, it’s lost.” Such insurance is available only through Lloyd’s of London. He said some Kelso homeowners had been paying Lloyd’s $500 a month for landslide insurance.

Because of all the nearby geologic activity, Portland State is a great place to teach and study geology, Burns declared. “I have no problem getting current events for my classes,” he said.