Folks' anecdotes will help scientists learn about quake

By Maureen M. Smith; Staff Writer

Sometime this week, researchers with the Minnesota Geological Survey will travel to western Minnesota to chat with people 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 survey. "We'll be trying to find out what people felt and experienced during the earthquake. What we really try to do is poinpoint the intensity of the earthquake."

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 may also reveal information about the gradual western movement of North America.

The tremor termed "light" by the U.S. Geological Survey, momentarily rattled a wide area around Wheaton, Minn., as well as other parts of western Minnesota, and eastern North and South Dakota about 8:25 p.m. Friday. It registered 4.1 on the Richter scale, slight compared to quakes that rock the West Coast. For example, the one that ripped up the San Francisco Bay area in October 1989 had a Richter reading of 6.9.

"The magnitude is very much in the ballpark of what we would expect; it's in the normal range for Minnesota," said Priscilla Grew, the survey's director. "It's not something p;eople should get too excited about."

Coincidentally, Grew was in earthquake-prone California for a meeting Friday, "I sort of expect small earthquakes in California, but I didn't expect one in Minnesota while I was out there," she said.

Several Twin Cities residents said they heard or felt the earthquake, Harold Dorland was at his home in West St. Paul watching television Friday night.

"I heard a noise that sounded like rolling thunder," he said. "I looked out to see if storm clouds were coming in, and I was really puzzled."

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. Many residents of western Minnesota also remember the 1975 quake centered on Morris, which registered 4.6, a jolt strong enough to break walls and windows.

Most of the quakes occurred along a line that runs from southwest to northeast through Morris, Alexandria, Staples and Nisswa, Minn. 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," Grew said. "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."

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

At any rate, earthquakes will probably happen again in Minnesota, Southwick said. But there's no predicting just when they might occur.

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