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Produced & Maintained by Idaho Mountain Express, Box 1013, Ketchum, ID 83340-1013 
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Copyright © 2003 Express Publishing Inc.
All Rights reserved. Reproduction in whole or in part in any form or medium without express written permission of Express Publishing Inc. is prohibited. 

For the week of October 22 - 28, 2003


Next week marks
20 years since
Borah earthquake

Seismic activity consistent
and continuing in Idaho

"Central Idaho is extremely vulnerable to earthquakes and has a great deal of seismic activity. The Borah Peak quake was just up the street from Blaine County."

STEPHEN WEISER, Assistant deputy director of mitigation for the Idaho Bureau of Disaster Services

Express Staff Writer

Historically, Idaho experiences a large earthquake about every 20 years, according to the Idaho Bureau of Disaster Services. Next Tuesday, Oct. 28, marks the 20-year anniversary of the Borah peak earthquake, Idaho’s largest recorded tremor.

At 7.0 on the Richter Scale, the Borah Peak earthquake was seismically significant, and it caused more property damage than any other temblor in the state’s history. It killed two children in Challis and inflicted approximately $12.5 million in damage in Challis, Mackay and areas in between.

"When it hit, it was like a clap of thunder or a sonic boom," a U.S. Forest Service spokesman told reporters following the quake.

Had it occurred in a heavily populated area, it could have killed thousands, according to a 1992 study by the Idaho Geological Survey.

The Borah Peak quake caused "spectacular" surface faulting and created a 21-mile-long zone of fresh scarps and faults on the southwest slope of the Lost River mountain range near the base of Mount Borah, Idaho’s highest peak.

In places, the earth shifted almost 9 feet in the blink of an eye, inducing rockfalls and landslides on the steep slopes of the Lost River Range, creating water fountains and sand boils, and even forming a temporary lake by shifting the water table. The earth’s rumbling was felt throughout the American and Canadian West, states a U.S. Geological Survey report.

"You could definitely feel it," said Bill Dyer, Blaine County building official, who was outdoors in Ketchum at the time. "It felt something like a big truck rumbling by."

Although Idaho is not known as an earthquake state, the Gem State’s residents should take heed from the example set by the Lost River Range in 1983.

"If you don’t get a lot of shaking, people don’t worry about earthquakes," said Stephen Weiser, assistant deputy director of mitigation for the Idaho Bureau of Disaster Services.

Along with frequent tremors and undetected small quakes, Idaho experiences a major earthquake about every 20 years, Weiser said, adding that it "isn’t like clockwork" and probably won’t occur in the same place every time.

The state’s numerous mountain ranges, hot springs and remnants of ancient volcanic flows are evidence of the formidable geologic forces that can cause the earth to move. In 1998 two small quakes occurred in the Sun Valley area, registering 2.50 and 3.0 on the Richter Scale. More have occurred in the last several years.

"Most of them are really small," Weiser said. "You’d have to be right on top of it. We have hundreds of those every year. There have been a number of threes over your way in the last couple years."

However, unless a quake occurs directly beneath a town or city, a measurement of 5.0 is generally needed to cause damage to buildings.

According to Disaster Services, earthquakes are measured in two ways.

Magnitude is calculated from the size of seismograph tracings. This rating, called the Richter scale, is a measurement of an earthquake’s energy at its epicenter and ranges from 1.0 at the low end to 9.0.

The other measurement, intensity, gauges the shaking caused by an earthquake in any one location away from an epicenter. It is represented by the modified Mercalli scale, in which a value of I represents the least intense motion and XII the greatest ground shaking. Intensity is not measured by machines but rather by the reactions of people and structures to the rumbling of an earthquake.

The Borah Peak earthquake, with an epicenter approximately 40 miles east of Ketchum and a Mercallis measurement of IX, noticeably shook the ground in the Wood River Valley.

"Central Idaho is extremely vulnerable to earthquakes, and has a great deal of seismic activity," Weiser said. "The Borah Peak quake was just up the street from Blaine County."

Blaine County is literally surrounded by faults, including a fault that runs along the western base of the Pioneer and Boulder mountain ranges.

To the northwest sit the Jakes Creek, Squaw Creek, Deadwood and Cat Creek faults, and to the northeast are the Lost River, Lemhi, and Laverhead fault systems.

The faults are part of the Idaho Seismic Belt, which, according to Disaster Services, is one of the largest earthquake zones in the United States in terms of energy release.

The Borah Peak quake, according to Disaster Services, "shattered windows, cracked walls, wrecked roads and killed two people. Five schools in the sparsely populated area suffered nearly $10 million in damages."

The damage to schools in Blaine County was minimal, consisting of minor cracks in floors and foundations.

The earthquake safety level of buildings is measured in structural and nonstructural terms.

According to Disaster Services, structural components, such as steel or reinforced concrete beams, hold a building up and resist forces caused by earthquakes. Nonstructural components, such as equipment, are not relied upon to stabilize a building and merely "go along for the ride" during an earthquake.

The 2000 International Building Code and, formerly, the Universal Building Code, established nationwide building standards for construction in seismic zones.

Dyer said the Wood River Valley also benefits because of sturdy construction designed to withstand significant snow loads.

"Because the building code is a minimum standard, it’s a balance," Dyer said. "And certainly some of the buildings in Blaine County are beyond minimum standards. We build for a potential snow load on our roofs, and that is intrinsic to the seismic design."

When people think of earthquakes and the damage the events can cause, they envision crumbling buildings, collapsing bridges and roads displaced by seismic rifts. Those are structural failures that occur in the most extreme earthquake scenarios. However, in an earthquake of moderate magnitude or intensity, nonstructural hazards can represent the significant threats.

According Dyer, buildings stay intact to some degree and foundations stay intact to some degree, but nonstructural elements can still pose hazards during quakes.

But as the day of the Borah peak earthquake’s 20th anniversary draws near, scientists maintain that the communities of Challis and Mackay emerged relatively unscathed.

"In spite of the real tragedy at Challis, with the little children being killed, I don’t think the people in Challis or Mackay realized quite how lucky they were, in the earthquake producing as little damage as it did," said Jim Zollweg, a seismologist with the University of Washington.

In a 1984 interview, Zollweg agreed with University of Utah seismologist Bill Richins that another sizeable earthquake is a distant possibility for Idaho. Another earthquake of a magnitude of 6.0 or 7.0 is "not at all unlikely," Richins said.



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