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
By GREG STAHL
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.