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March 29, 2015
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Not if, but when and how big?

(page 1 of 4)

Imagine: You’re awakened in the wee hours of the morning by a low-frequency roar coming from … everywhere. Your house moves all around you, its walls bending unnaturally. You leap from bed only to be thrown to your knees. You hear breaking glass, tumbling objects, and panicked voices in other rooms. Seconds seem like minutes. It’s not stopping, and you wonder if the old place will hold together as you scramble for a safe spot. Finally, stillness returns and you take stock. You’re in one piece, but what now? Is anyone hurt? Will you need medical help? Is the house too badly damaged to remain inside? But the power and heat are out, and it’s freezing outside. Is help even available?

Teton Valley residents seldom ponder such a scenario, as few have exper-ienced more than minor tremors. But the jagged Teton Range should be a constant reminder that a major earthquake is always a possibility. After all, those mountains are the product of violent upward thrusts of a major fault located roughly twenty miles east of the valley. Also nearby is one of the most active geothermal features in the world: the Yellowstone “hotspot.”

 In light of this geologic reality, community leaders and regional scientists are working to educate and prepare eastern Idaho residents for the inevitability of future earthquakes. It’s not a question of if, but of when and how big?

Scientists say our region could experience earthquakes even more powerful than some of the fairly recent California events [see “Been There, Felt That,” page 39]. “Damaging earthquakes have occurred in Idaho and may again at any time in the future,” says Bill Phillips, a research geologist with the Idaho Geological Survey. “Large earthquakes happen here infrequently—but they do happen—and people forget the lessons learned by their parents and grandparents.”

The largest seismic event centered in Idaho in historic times was the 6.9- magnitude Borah Peak earthquake of 1983, which resulted in two fatalities and more than $12 million in damage in the Challis-Mackay area. The (Montana) epicenter of the 1959 Hebgen Lake earthquake—at magnitude 7.3 one of the most powerful ever recorded in the lower 48—was just seventy miles from Driggs. Twenty-eight people died, and major highway and landslide damage occurred.

According to the U.S. Geological Survey, the probability of a 6.0 or greater quake within thirty miles of Driggs in the next thirty years is between 15 and 20 percent. Both the Teton Fault and the Yellowstone hotspot present obvious risks to Teton Valley.

The Teton fault is a “normal,” or vertical-displacement, fault, related to the spreading of regional basin-and-range topography. Somewhere around six to nine million years ago, earthquakes began pushing the Tetons upward relative to Jackson Hole, at an average rate of one foot every three or four centuries. Doesn’t sound like that much? Well, geologists believe the fault has produced earthquakes up to 7.5 magnitude throughout its history—enough force to lift the range upward three to six feet in a single event.

Yet the Teton fault has been relatively quiet in historic times. The last local quake exceeding 7.0 magnitude is thought to have occurred around 4,800 years ago. But read on.

The Yellowstone hotspot is caused by a stationary plume of molten material rising from deep within the earth’s mantle. This plume periodically erupts through the earth’s crust as volcanic activity. The overlying crust, however, is not stationary. It floats about at the surface in the form of large tectonic plates. Evidence of extensive volcanic activity across the Snake River Plain indicates a gradual southwestward movement of the North American tectonic plate relative to the hotspot (see illustration, page 38).

The Yellowstone caldera, a thirty-by-forty-five-mile crater taking in much of the park—including most of Yellowstone Lake—is the remnant of a super-eruption that took place “just” 640,000 years ago (which is, in fact, the blink of an eye on the geologic time scale). It was the most recent in a series of cataclysmic volcanic events extending across the Snake River Plain. Smaller but significant eruptions have occurred in Yellowstone as recently as 13,800 years ago.

Major geothermal activity in Yellowstone continues, as is obvious to anyone who has watched Old Faithful erupt. A $2.3 million, seventeen-year study by the University of Utah revealed that ground movements and energy produced by the hotspot are much greater than once thought. This movement regularly creates “swarms” of small earthquakes.

From December 2008 through January 2009, for example, around nine hundred small earthquakes occurred in the Yellowstone-Teton region. Precise GPS-enabled sensors show that pressure from magma and hot water cause the caldera to “huff and puff” dramatically, yet no volcanic eruptions have occurred (so far). For example, portions of the caldera floor sank 4.4 inches between 1987 and 1995, and others rose a staggering seven inches between 2004 and 2007.

Counterintuitively, Yellowstone’s dramatic ground movement may actually explain the lack of recent earthquake activity along the Teton Fault. According to Professor Robert B. Smith, leader of the University of Utah study, “The textbook model for a normal fault is not what’s happening at the Teton Fault. The mountains are [now] going down relative to the valley going up. That’s a total surprise.”

Smith believes that pressure from the bulging Yellowstone region is pushing the Tetons and Jackson Hole together, reversing the fault’s normal movement pattern. But this may also be causing pressure on the fault to build to a critical level. Smith believes over the long term, upward displacement of the Tetons will eventually prevail, possibly causing a catastrophic earthquake.

While Yellowstone and Jackson Hole remain a focus of geological research, scientists have recently turned their attention to Teton Valley. Over the past year, geologist Phillips mapped local soil and rock conditions that influence liquefaction and shaking intensity during earthquakes. Liquefaction occurs as loose, water-saturated soils quickly lose strength during the shaking of an earthquake, causing man-made structures to sink and/or collapse. San Francisco’s Marina District in the 1989 Loma Prieta quake is a classic example: buildings constructed on fill collapsed, while nearby structures occupying firmer ground suffered no damage. Phillips’ map shows an increased likelihood of liquefaction in areas near the Teton River, where sandy alluvial soils coincide with a shallow water table.

Phillips also produced a map classifying local surface geology based on how shear waves (side-to-side energy) pass through it. “We’re looking at the shear component because that’s what causes buildings to collapse,” he says. Basically, shear waves travel slower through softer, less-consolidated materials, amplifying shaking—and putting buildings at higher risk. “It’s like a bowl of Jell-O versus a block of wood,” he says.

Both of these maps may be important for planning purposes, like assessing building sites for critical public structures. While Phillips rates Teton Valley’s seismic hazard as “moderately high,” he says the good news is that housing in the valley includes many newer homes and wood-frame structures that should hold up fairly well in a major quake.

Older buildings, especially those built of unreinforced masonry, may be more at risk, since earthquake-resistant building standards are a relatively recent development. “That goes back to the beginning of the Building Department in 1994, when we started enforcing seismic design,” says Tom Davis, Teton County’s building inspector. “And that has evolved. We’re now in the sixth [International Building] codebook. It’s pretty stringent.”

Greg Adams, emergency management coordinator for Teton County, says the old county courthouse, which holds the county’s emergency dispatch center, is one major concern. If a quake damaged or destroyed that building, coordination of emergency response could be seriously hampered. Fortunately, a new county law enforcement and dispatch center, built to modern earthquake standards, is slated for completion in 2014 (at the site of the old school administration building/learning center at Buxton and North Main in Driggs).

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