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October 25, 2014
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Not if, but when and how big?

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).


The county’s elementary schools, built in the 1940s and ’50s, also pre-date modern seismic standards. Monte Woolstenhulme, superintendent of Teton School District 401, reassures parents that school buildings are inspected annually by state officials and upgraded as needed. “They are very well-built, well-constructed facilities” he says; the buildings have withstood significant events like the 1959 Hebgen Lake earthquake. And the school district has initiated a program of practicing earthquake drills. As part of Earthquake Awareness Month last October, for example, Tetonia Elementary participated in an earthquake scenario drill including live response from the Teton Fire District and local Emergency Medical Services.

Adams says Teton County is coordinating with the Red Cross, public works, and emergency agencies across the region to develop disaster-response plans. He says the new Greater Yellowstone Response and Planning Group will involve eight to ten counties, as well as the National Park Service. In April, emergency agencies from Teton County and surrounding Idaho counties participated in a statewide earthquake drill, including a Palisades Dam failure scenario. Adams has also applied for funds to perform digital earthquake damage analysis, using data from Bill Phillips’ recent geological studies.
Individual initiative is an equally important component of community earthquake readiness, Adams notes. “Mostly, we want to encourage people to be prepared, get a kit, make a plan, and get involved.” Home preparedness for an earthquake includes simple, common-sense precautions such as having shoes and a flashlight readily accessible, not placing heavy objects and windows over beds, and installing flexible gas lines for appliances (see page 42).
 In a worst-case scenario, a severe earthquake could strike in the dead of winter, during bad weather and closed highways. With a prolonged power outage, many residents would lose their heat source. If the airport were damaged, getting help and supplies into the valley would prove problematic. Residents could be largely on their own. The more people who are individually prepared, the further public emergency resources will go.

With good planning, improved building standards, and proper emergency coordination, the worst of an earthquake disaster could be avoided. There is nothing we can do to stop the awesome power of an ever-shifting earth, but we can minimize the damage, fear, disorientation, and displacement often associated with an earthquake.



I can tell you from experience that witnessing a major earthquake is an awe-inspiring, disorienting, and frightening experience. Back in 1971, I awoke early one morning when my body hit the floor. The initial jolt of the San Fernando, California, earthquake—6.6 on the Richter Scale—had thrown me from bed. Brilliant flashes lit up the sky (which turned out to be power transformers exploding), and it sounded like the world was ending. My initial thought was that nuclear Armageddon was upon us. But as the shaking continued, I realized what was happening.

The quake lasted a full, terrifying minute. Our wood-frame house suffered little damage, but the power outage lasted for hours. The aftershocks were nerve-wracking. Two hospitals collapsed, freeway interchanges fell to the ground, and older schools across the city suffered debilitating damage. The concrete lining of a local earth-fill dam disintegrated, leading to the rapid evacuation of 40,000 residents. In the end, sixty-five people lost their lives.

Years later, as I rode my bicycle through Golden Gate Park, the Loma Prieta quake struck the San Francisco Bay Area. Game three of the 1989 World Series was about to begin, so effects of the magnitude 6.9 earthquake were seen live by millions on national television. In lieu of aerial photography of the ballpark, the Goodyear Blimp provided viewers images of fires and damage across the city.

Outside and in the daylight, I saw the full power of the earth’s movement. Street pavement rolled like viscous liquid in an undulating wave. Light poles swung wildly to and fro as cars bounced like lowriders showing off their hydraulics. The driver in the car next to me screamed into her cupped hands.

Damage was light in many areas—but places like the Marina District, built upon filled land adjacent to the bay, were devastated from liquefaction of the soils. Forty buildings in Santa Cruz, near the epicenter, were completely destroyed. A portion of the double-decked Nimitz Freeway in Oakland collapsed, and hundreds were initially feared dead. Luckily, the World Series game had altered traffic patterns, dramatically reducing casualties. Still, the quake caused sixty-three deaths and $6 billion dollars’ worth of damage.  – R.M.



Before an earthquake

• Develop a home earthquake plan so that you know ahead of time what to do during and after an earthquake.
• Conduct earthquake drills with your family or coworkers. Locate safe spots (e.g., under a sturdy table, bench, or a load-bearing doorway), and identify danger zones (e.g., near windows, glass, outside doors and walls; and anything that could fall, such as lighting fixtures or furniture).
• Develop a plan for reuniting all family members after an earthquake occurs.
• Choose an out-of-state friend or relative who separated family members can call after the quake to report where they are and how they’re doing. Oftentimes during a disaster you won’t be able to call across the street, but you might be able to call out of state. And always keep your cell phone charged.
• Keep supplies on hand, including food and water for 96 hours (a normal adult needs at least ½ gallon of water to drink per day), a flashlight with extra batteries, a portable radio, a fire extinguisher, and tools (i.e., shovel, screwdrivers, hammer).
• Store heavy and breakable objects on low shelves. Weed killers, pesticides, and flammable products should be kept on bottom shelves or in closed cabinets with latches. Chemicals will be less likely to create hazards if they are stored in lower, confined locations.
• Secure bookshelves, water heaters, and tall furniture to wall studs. Install latches on all cabinets, and anchor overhead lighting fixtures. Secure items that might fall, such as televisions.
• Have a licensed professional install flexible pipe to gas appliances to avoid gas leaks.
• Move beds away from windows, if possible.
• Move or secure hanging objects over beds, couches, and other places where people sit or lie down.
• Keep shoes and a flashlight under the bed. Keeping shoes under the bed ensures quick access to prevent cutting feet on glass and reduces the risk that glass could fall into them.

During an earthquake

• Drop, cover, and hold. Move only as far as necessary to reach a safe place. Most people injured in earthquakes moved more than five feet during the shaking.
• If indoors, stay there. Many fatalities occur when people run outside, only to be killed by falling debris from collapsing walls. It is safer to stay indoors until the shaking stops and it is safe to exit. When going outdoors, move quickly away from the building to prevent injury from falling debris. Extinguish candles, matches, or other open flames in case of gas leaks.
• If outdoors, find a spot away from buildings, trees, streetlights, power lines, and overpasses. Drop to the ground and stay there until the shaking stops.
• If in a vehicle, pull over at a clear location and stop. Stay in the vehicle with seatbelt fastened until the shaking subsides.

After an earthquake

• Check for injuries. Protect yourself by putting on long pants, a long-sleeved shirt, sturdy shoes, and work gloves.
• Check others for injuries. Apply first aid if necessary.
• Check for gas and water leaks, and broken electrical wiring or sewage lines. If there is damage, turn utility off at the source.
• Stay away from downed or damaged utility lines.
• Turn on a battery-operated radio or TV for emergency instructions.
• Do not use the telephone unless there is a serious injury or fire.
• Be cautious when opening closets and cupboards. Displaced objects may fall when the door is opened.
• Be prepared for aftershocks. Each time you feel one—drop, cover, and hold.
• If you evacuate, post a message telling family members where you can be found.

If you have any questions regarding earthquake preparedness, call the Teton County Emergency Management Office at 208-354-2703.

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