Earthquakes in Kansas a real threat
At the base of Tuttle Creek Reservoir, a 20-ton clamshell excavator removes giant buckets of earth, making way for huge concrete walls.
The amount of cement in those walls could pave a sidewalk between Tuttle Creek just outside Manhattan, Kan., and Washington, D.C.
This $150 million project is all the proof you need that earthquakes are a real threat in Kansas.
Without these walls, catastrophe could strike, the U.S. Army Corps of Engineers has decided. Twelve miles east of the dam is the Humboldt Fault Zone, a series of north-south fault lines that run from Oklahoma City to Omaha, Neb.
An earthquake in this region – not much greater than one that shook southern Illinois earlier this month or was felt in Manhattan more than a 140 years ago – could mean a breach in the dam.
The worst-case scenario? Water pours out at 300,000 cubic feet per second – five times faster than Kansas River floodwaters in 1993. In two to six hours, water would inundate downstream Manhattan. The local shopping mall would be under 10 feet of water, the levee would fail and water would lap at the outskirts of Kansas State University. In the water’s path are 13,000 people and 5,900 homes.
The scene is like something out of a disaster movie and not one all residents believe will happen.
But geologists and engineers say the threat is real.
“They are very possible in the Midwest, and they occur on a regular basis,” said Kathleen Lust, the Corps’ resident engineer on the Tuttle Creek Dam Safety project.
In the Fault Zone
Just 15 miles from Manhattan is Wamego, home to the first – and largest – earthquake ever reported in Kansas. On April 24, 1867, the earth moved as far away as Dubuque, Iowa. In Manhattan, people rushed to the streets and a two-foot wave was spotted on the Kansas River. Doors and windows rattled in Lawrence. And a Leavenworth man was shaken off a load of hay. The earthquake was estimated at 5.1 on the Richter scale.
More than 25 earthquakes have been felt since.
Many Kansas earthquakes are along the Humboldt Fault Zone. However, Kansas’ largest earthquake in the past 50 years – one that registered around 4 on the Richter scale – was along a fault line that runs from Wichita to the Black Hills.
“It doesn’t matter if you are a native Kansan or not, most people are surprised to find out that there are, from time to time, small earthquakes,” Don Steeples said.
Now a vice provost of scholarly support at Kansas University, Steeples has spent a good part of his career as a geophysicist studying earthquakes in Kansas. For 12 years, he worked to record every earthquake along the Humboldt Fault Zone. He found a couple of shakes every year, but nothing more than a magnitude of 2.7, which is almost too small to feel.
However, he believes the Humboldt Fault Zone has the potential to produce earthquakes in the 6 to 6.5 range.
“That’s a pretty good-size earthquake,” he said. “You know it wouldn’t cause widespread devastation to homes. Damage? Yes. But flattening homes, killing lots of people, not a magnitude of 6 or 6.5.”
Large structures – power plants, dams and skyscrapers – are most at risk, said Rex Buchanan, Kansas Geological Survey deputy director.
“For the most part, these are not issues that affect everyday people,” he said. “Yeah, you might feel an earthquake, but it is not exactly something that I lie awake at night worrying about.”
Officials were aware of the Humboldt Fault Zone before Tuttle Creek was built in the 1950s. What wasn’t as obvious was the threat powerful earthquakes posed to earthen dams.
That changed in 1971, when a 6.7 magnitude shook California’s Lower San Fernando Dam. According to the U.S. Geological Survey, a landslide carried away the dam’s crest and upstream concrete facing, leaving a narrow wall of dirt. Had the dam been at maximum capacity, it would have breached and flooded the valley below. More than 80,000 people were forced to leave the valley.
Beginning in the 1980s, the Corps of Engineers studied Tuttle Creek. It determined an earthquake between 5.7 to 6.6 would cause sand underneath the dam to liquefy into quicksand, causing the dam to spread out and the top to drop up to three feet.
While the dam wouldn’t fall below lake level, a large earthquake would cause water to seep through the dam, eroding it internally until it failed.
To solve the problem, the Corps has engineered a project unlike any in the world.
More than 350 concrete walls – 45 feet long, 60 feet deep and 4 feet wide – are being dug at the base of the dam to replace the earthquake-susceptible sand and stabilize the toe of the dam. The project is on track to finish by September 2010.
If an earthquake were to occur before then, the dam has been fitted with elaborate sensors that can set off an alarm system. Sitting below the dam are six sirens that will sound a warning alerting people to head for high ground.
Dam-hurting earthquakes have a probability of occurring once every 1,800 years, the Corps said.
“There are still people that say, we are in Kansas. There are not earthquakes here. It is not going to happen here. I hope they are right,” said Brain McNulty, the Corps’ operation manager at Tuttle Creek. “But I am not willing to take the risk, and the organization is not willing to take the risk to jeopardize that many people downstream.”