For the week of December 16 thru December 22, 1998  

Blaine County ready as possible for the Big One

Major fault systems crisscross this hot spot


By KEVIN WISER
Express Staff Writer

Idaho will probably never be nicknamed the Earthquake State, but the potential for quakes here is much greater than many people realize. In fact, the 1983 Borah Peak earthquake was one of the largest ever in the lower forty-eight states.

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

Historically, Idaho experiences a large earthquake about every 20 years, along with frequent tremors and undetected small quakes. The state’s numerous mountain ranges, hot springs, and remnants of ancient volcanic flows are evidence of the forces that cause quakes. This year two small quakes occurred in the Sun Valley area registering 2.50 and 3.0 on the Richter Scale.

According to the IBDS, earthquakes are measured in two ways. Magnitude is calculated from the size of seismograph tracings. This Richter scale is a measurement of earthquake energy at its epicenter and ranges from 1.0 (weak) to 9.0 (strong). Intensity measures the shaking caused by an earthquake in any one location away from the epicenter. It is represented by the modified Mercalli scale—a value of I representing 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 shaking during an earthquake.

The 1983 Borah Peak earthquake, which registered 7.3 on the Richter scale, was felt here in the Wood River Valley. The quake’s epicenter was approximately 40 miles north-northeast of Ketchum and occurred along the Lost River Range Fault System, which runs south-southeast down from Challis and bisects the northeast corner of Custer County.

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

Blaine County is literally surrounded by faults.

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.

Those faults are part of the Idaho Seismic Belt, which, according to the IBDS, is one of the largest earthquake zones in the United States in terms of energy release. (See map of Idaho showing areas of relative seismic shaking hazards.)

The Borah Peak quake, according to the IBDS, "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 the IBDS, structural components, such as steel or reinforced concrete beams, hold a building up and resist forces caused by earthquakes. Nonstructural components, such as equipment and contents, are not relied upon to stabilize the building and literally "go along for the ride" during an earthquake.

The Uniform Building Code—a nationwide agency that sets standards for construction in seismic zones—ranks Idaho among the best in the nation in regard to earthquake safety.

According to Ketchum building inspector Dennis Wheeler, Hemingway Elementary, for instance, is up to code as far as UBC structural standards go.

However, in a letter from the IBDS, Weiser expressed concern that the Blaine County School District had not taken advantage of the opportunity to improve the safety of schools by way of grants for nonstructural seismic retrofits, available from the IBDS Hazard Mitigation Grant Program.

According to Jonathan Perry, coordinator for the IBDS Hazard Mitigation Grant Program, there is more than $700,000 left in the $1-million fund, and many schools in the state have not made nonstructural improvements.

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, it is often the nonstructural hazards that represent the greatest threat.

According to Bill Dyer, Blaine County building official, often when large quakes occur, buildings stay intact to some degree and foundations stay intact to some degree, but nonstructural elements still pose a hazard.

Nonstructural precautions for schools include securing file cabinets to walls; securing desktop computers; securing shelves and their contents and removing heavy objects from top shelves; securing heavy equipment and display cases; and securely mounting fire extinguishers and heavy wall-mounted objects.

Blaine County School District Superintendent Phil Homer said schools are required to take care of nonstructural requirements. He said the county’s schools are safe both structurally and non-structurally.

Ketchum architect Robert Hart, who has been involved in emergency disaster planning in the county, agreed that the schools are "relatively safe."

However, Hart suggested that that might be an oxymoron when used in the context of the power of natural forces. He said an earthquake greater than anything anticipated could occur, and overwhelm all building codes and engineering standards.

Though devastating, earthquakes serve to remind us that the forces of nature— at their most extreme— have the potential to defeat all man’s technological and engineering advancements. At that point, our understanding must give way to awe and wonder.

 

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