Morris Tribune
Thursday, June 10, 1993

Earthquakes happen periodically in 'Zone'

"Morris just happens to sit in or on the Great Lake Tectonic Zone," and earthquakes along the zone can be expected every 15 to 20 years, according to Jim Cotter, associate professor of geology at UMM.

Cotter explained that the area is called a "zone," not a "fault," since the exact location is hard to pin down. It's more of a band, about 30 miles wide, from Duluth westward across the state through Little Falls, Sauk Center, Alexandria, Morris and Ortonville.

He said the zone is "situated purely on the basis of earthquakes that have occurred."

The Great Lake Tectonic Zone continues into South Dakota, and then, Cotter said, "you lose sight of it and pick it up again in Nebraska."

He said that it is probably an old fault system in the range of two billion years old that at one time was very, very active -- comparable to California.

The zone is now in its "last gasp stages" with small earthquakes -- magnitude 4 earthquakes, Cotter said.

For the Upper Midwest, that's "pretty active," he said. Earthquakes occur along the zone every 20 or so years.

But, compared to some more active areas of the globe, it's pretty tame.

Cotter said he would expect an earthquake somewhere in the zone every 15 to 20 years.

During the late 1800s, quakes occurred in Minnesota at 5 to 10 year intervals.

He said it was unusual that the last two earthquakes occurred in essentially the same spot. It was an odd adjustment for them to occur one on top of the other.

The epicenter of the July 9, 1975 earthquake was calculated at 6 to 10 miles west and 2 to 3 miles south of Morris. The epicenter of Friday's earthquake was calculated at 22 miles northwest of Morris.

The magnitude of Friday's earthquake was 4.1. Cotter said such earthquakes -- 4.1 to 4.9 -- are termed "minor." Globally, there are 6200 minor earthquakes a year.

It's usually in the magnitude 5 to 5.9 that damage occurs. There are 800 earthquakes around the world in that range every year.

He said that in California, a 4.1 magnitude quake wouldn't upset anyone; they're used to it. In an area like New York City, he said "it would certainly open people's eyes."

Friday's quake was probably felt in a 50 mile radius of the epicenter. Cotter commented that he couldn't believe people in the Twin Cities felt it, although television reports said some did.

Local awareness of the quake depended on building structure. Cotter said someone with a lot of china in a china cabinet would be more aware of it than someone with bulky furniture.

And people in taller buildings would have been more aware of the quake. He said that when the ground starts to vibrate, that motion is transferred to the building, which starts to sway. The ground starts and the top of the building makes up for it. It's called "harmonic motion," he said.

In a 5.5 to 6 magnitude earthquake there could be damage to taller buildings -- 10 stories. Cotter said that after the vibration starts, the earth stops moving, but the building doesn't.

Aftershocks

He said aftershocks are usually a smaller magnitude than the original earthquake, and it's hard to feel an earthquake below magnitude 4, so it's unlikely any aftershocks would be felt. But he added, that none has been recorded for Friday's quake.

Eighteen years ago, teams from the U.S. Geological Survey in Golden, CO came to Morris to measure aftershocks from the 1975 quake. Cotter said he hasn't heard about anyone coming out from there, or from the Minnesota Geological Survey.