West Central Tribune
June 7, 1993

ST. PAUL (AP) -- Researchers looking for ancient fault lines will travel this week to western Minnesota in search of clues from anecdotes about how Friday's earthquake felt.

"Did their teacups rattle, did something fall off of the wall, did windows break?" said G. B. Morey, an associate director and chief geologist with the Minnesota Geological Survey.

"We'll be trying to find out what people felt and experienced during the earthquake. What we really try to do is pinpoint the intensity of the earthquake," he said.

Such data will help researchers to better determine the exact location of ancient fault lines in the normally stable, rocky crust that underlies Minnesota. The data also may reveal information about the gradual westward movement of North America.

The tremor termed "light" by the U.S. Geological Survey momentarily rattled a wide area around Wheaton, as well as other parts of western Minnesota and eastern North and South Dakota about 8:25 p.m. Friday. It registered 4.21 on the Richter Scale, slight compared to quakes that rock the West Coast.

Minnesota has been the site of 16 earthquakes since 1860, including the record-holding 1860 quake in Long Prairie, which researchers estimated to be 5.0.

Most of the quakes occurred along a line that runs from southwest to northeast through Morris, Alexandria, Staples, and Nisswa. This area is part of the Great Lakes Tectonic Zone, the major fault line in the state. As the crust of the North American plate gradually moves westward, the movement stresses weak seams in the Earth's crust.

"Some active fault lines from billions of years ago have stabilized now," said Priscilla Grew, the Minnesota Geological Survey's director. "But where there are joints between rocks of different ages, they tend to quake a bit. It's sort of like creaking in the floorboards."

Another factor thought to cause earthquakes in the Midwest is "glacial unloading," said David Southwick, assistant director at the survey. This is the gradual rebounding of parts of the Earth densely compacted under glaciers about 15,000 years ago.

Anyone with information about the earthquake may write to the Minnesota Geological Survey at 2642 University Ave. W., St. Paul, Minn., 55114.