Morris Tribune
June 17, 1993

Background on fault . . .

The fault zone in this area begins in Big Stone County and goes east to the central part of the state.

Val Chandler of the Minnesota Geological Survey pointed out the fault zone on a map at a news conference Tuesday in Graceville.

The map was a seromagnetic anomaly map created from sensors in p;lanes flying over the area. Chandler said there is a magnetic field change along the fault zone from the gneiss rock to the south of it and meismorphic rock with a lower intensity magnetic field to the north.

He said there had been 16 -- now 17 -- quakes along the zone. The two largest events, the Morris quake in 1975 and the Staples quake in 1917, occurred along the line.

"It shouldn't be too shocking that there are faults in Minnesota," Chandler said. The process of continent building creates stresses that move and form faults. That process in Minnesota is long since dead, but it can be jostled into activation, he said.

This area, though is nothing like the New Madrid Zone in Missouri. Chandler said probabilitly of an earthquake here is low, but not zero.

Given enough time, he said, any part of the continent can generate an earthquake.

Earthquake theories

Chandler said there are two theories about why earthquakes occur in this area. One is the earth is still slightly rebounding from the weight of the last ice age. The other theory is plate tectonics or that North America is moving to the west, away from the Atlantic Ridge -- continental drift. He said that movement sets up subtle stress in the earth's crust that can "pop loose" with an earthquake.

The stress from continental drift is not as great here as it is in California, though.

Of the 17 earthquakes in Minnesota, five have been in the fault zone that goes across the state. The 1975 earthquake near Morris was just outside that zone, he said, but the zone as shown on the acromagnetic anomaly map is offset right at Morris.

Chandler said maybe something else is happening in Morris. One speculation, he said, is that there could be two fault zones.

He said the fault zone, the Great Lakes Tectonic Zone, was formed about two and a half billion years ago.

Collis, in Traverse County, where the epicenter of the June 4 earthquake is probably located, is just north of the fault zone. But Chandler said the fault dips to the north, under the surface, so it is still probable that it is related to the fault.

Chandler said the June 4 earthquake probably had a shallow focus -- somewhere between the surface and six miles deep. He said that with a more shallow focus, the earthquake is felt over a wider distance. It's typical in this area for earthquakes to be shallow. And, he said, reports of the earthquake were felt from the Twin Cities to southern Minnesota.

The earthquake probably involved only inches of movement, compared to the last major San Francisco earthquake that involved about 20 feet of movement over 10s of kilometers.

He said the earthquake itself probably only lasted a second. The surface waves, though, were felt for 10 to 20 seconds. It's the two types of surface waves that do most earthquake damage, Chandler said.

There's been no sharp focus of damage that would help pinpoint the epicenter, he said. There have been some reports of cracks in walls and one report that a mirror fell off the wall.

Aftershocks

Some of the questions Chandler and his team of University of Minnesota students will be asking people in the area will be about similar events they might have felt since the quake -- aftershocks.

He said if they get a cluster of comments, it might indicate an aftershock. But such aftershocks are rare for earthquakes of this type. They're usually "one pop affairs," he said.

Chandler noted that the U.S. Geological Survey in Golden, CO had sent a team to measure aftershocks after the 1975 earthquake, but didn't this time.

Chandler said a magnitude 4 earthquake could be expected about every 10 years in this part of Minnesota. A magnitude 4.5 quake will occur about every 30 years, and one at 5 would be about every 90 years.

It's nothing like California, though, he said. "They tend to laugh at events like this." He added that he didn't know whose name would be used to designate this quake -- Dumont, Graceville, or Collis.