Land of lakes has its quakes
Minnesotans have felt 2 minor jolts this past year

by Laura S. Ballman; Staff Writer

When Kevin Mason was awakened during the night of Feb. 9, he had no idea that the Yellow Medicine Shear Zone was rumbling several miles underground.

"When it hit, it made a big bang in the house. It woke me up. It was that loud," said Mason, who lives 20 miles from Willmar in west-central Minnesota. At first he thought the noise might be a tree falling outside his house. But then he discovered cracks running across his bedroom wall.

"I wasn't really scared, but I was wondering what happened," he said.

What happened was an earthquake, according to a University of Minnesota researcher who, more than 10 weeks later, still wants to hear from people with similar experiences.

The quake was the second in Minnesota in less than a year. The other was felt near Graceville on June 4.

"Any piece of the continent is full of faults," said Val Chandler, a geophysicist at the Minnesota Geological Survey.

When Mason showed up for work the next day at the Willmar Civic Center, his colleagues told similar tales of a nighttime disturbance.

"It was like in the winter when it's real cold and the boards in your house contract and will make a loud noise. But it wasn't even very cold that night, so I knew it couldn't be that," said Ralph Nelson, who also works at the Civic Center.

Things were not right at work, either. Nelson said the normal power flow to the compressor at the Civic Center ice arena was interrupted.

The group decided to call scientists at the University of Minnesota. Enter Chandler, who charted the Willmar anecdotes, such as rattling and odd noises, and found that the incident registered 3.1 on the Richter scale. That's a relatively weak quake that probably didn't wake up many people.

The personal accounts of the earthquake do not seem shaky to Chandler. "Usually you can tell if they are pulling your leg, "he said. "I've not seen anything that looks suspicious."

After collecting 23 reports across 4,400 square miles, Chandler noted a cluster pattern, indicating that the epicenter was 3 miles north of Granite Falls.

The oldest Minnesota faults formed more than 2 billion years ago, but they still can be jostled into activity. Chandler suspects that the main culprit in the Feb. 9 case was a fault called the Yellow Medicine Shear Zone.

"The one mystery still is what time it happened," Chandler said. The closest seismograph, in Rapid City, S.D., registered waves at 2:45 a.m., but reports from the Willmar area came up to 30 minutes later.

Chandler said the discrepancy means that people were too tired to notice the exact time they awoke or that the quake involved more than one tremor.

The June 4 earthquake was widely documented near Graceville, a town of 671 people about 80 miles northwest of Willmar near the South Dakota border. Most of the several dozen reports, collected from a 26,800-square-mile area, suggest that it happened about 8:25 p.m. It measured 4.1 on the Richter scale: Dishes fell, concrete barn foundations cracked and knicknacks on tables toppled.

"It's kind of a novel thing to happen here," said Andrea Wulff, Graceville city clerk.

But according to University of Minnesota scientists, state residents can expect an earthquake registering at least 3 on the Richter scale every three years, and a magnitude-3.5 quake every five years.

The most spectacular thing about the Graceville quake was the noise near the epicenter, several miles northeast of town, said Chandler.

People compared it to thunder or sonic boom, he said. "A lot of people thought the local crop-duster crashed."

Wulff was preparing for her daughter's graduation party at a hall when the earthquake hit.

"It felt like the whole building was shaking, and you could hear it. We went outside, and the tremor was still there," she said. "So we went back inside, and we felt the aftershock." There was "that feeling of uncertainty, not knowing what it was," Wulff said.

Anyone with information about the quakes can contact the Minnesota Geological Survey, 2642 University Av., St. Paul, MN 55114-1057.