Star Tribune
http://www.morris.umn.edu/earthquakes/articles/jan301990.html
Star Tribune
January 30, 1990
Why no big earthquakes here? Location, location, location
By Jim Dawson; Staff Writer
In 1975, the ground under Morris, Minn., shook with what must have seemed an incredible
force to Midwesterners.
The jostling was the result of a 4.9-magnitude earthquake. That's small by California
standards but strong enough to excite one man to the point where he fell out of a haystack
and injured himself.
That's about as dramatic as earthquakes get in a region made of "old, cold continental
crust" that dates back 3.5 billion years in some parts of Minnesota. As steady and solid as
the miles of rock under the Upper Midwest is, however, Saturday's small earthquake near
Ogala, S.D., and a similar small tremor in that state in November, are reminders that the
ground in the region does occasionally shift and shake.
There have been 16 earthquakes in Minnesota since 1860, and five of them are recent
enough (beginning in 1964) to have been recorded by scientific instruments. Estimates of
the strength of the others, including 1860's record-holding magnitude-5 quake in Long
Prairie, are based on estimates by researchers who have looked at damage and read
accounts of what happened.
Why is Minnesota and the rest of the Upper Midwest immune from "the big one" -- an
earthquake of 6 or greater magnitude?
The simple answer is position. The state is in the middle of the North American continent
and built on some of the oldest rocks on the planet. They are old because, being in the
middle of the continent, the region has not been subjected to the intense grinding and
breaking that occurs along the West Coast, where two continental plates are colliding.
There are many ancient faults, or cracks, in Minnnesota's bedrock, including a massive
formation called the Mid-continent Rift. The rift, said Val Chandler, a geophysicist with
the Minnesota Geological Survey, is the scar left from well over a billion years ago when
North America tried to rip apart.
Chandler said the rift, which runs from southeast Minnesota up to and under Lake
Superior, "was the last big event" in the state. "The continent tried to pull apart," he said,
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"and there was evidence of an ocean basin beginning to form."
But for reasons still unexplained, the rift stopped and the continent remained in one piece.
"It seems to be a bit of a dud," he said.
While that rift apparently isn't the cause of the small earthquakes in the Upper Midwest, it
seems to be related to the New Madrid Seismic Zone between St. Louis and Memphis,
Tenn.
That area has been the site of a number of very large earthquakes in the early 1800s, which
may have been greater than 8 on the Richter Scale. The scale is logarithmic, meaning that
an increase of one number, say from magnitude 7 to 8, represents a tenfold increase in an
earthquake's power. The recent one near San Francisco was a 7.1, compared with the great
San Francisco earthquake of 1906, which was a magnitude 8.3.
The New Madrid zone seems to be a unique geologic formation that somehow focuses
stress in one area, resulting in large earthquakes in a place where, judging by location in
the middle of the continent, they shouldn't occur.
While there is no such stress-focusing area in Minnesota, there are many small cracks in
the state, particularly in the center. At one point geologists believed that one zone of faults,
known as the Great Lakes tectonic zone, was responsible for some of Minnesota's small
earthquakes, but Chandler said that idea has faded.
So what causes Minnesota's earthquakes?
The leading theory holds that as the entire North American continent drifts westward,
stress builds up across the entire land mass. It is a very slow process, with the continent
moving only about an inch a year. The drift, Chandler said, occurs at about the same rate
that fingernails grow. The result, he said, is "subtle but pervasive stress, and the old faults
are weak zones that can be jostled."
So as California grinds westward over the Pacific plate, which is moving east and north,
stress builds up across the United States, including Minnesota. "The real action is where
the plates are rubbing against each other," Chandler said, noting the recent earthquake near
San Francisco and volcanic activity in Alaska.
In Minnesota and the eastern two-thirds of the Dakotas, it may be this continent-wide
stress that causes "chattering," Chandler's word for the region's small quakes.
That hasn't changed much in the last billion years, he said, and "As far as we know, there
is nothing here to produce the big one."
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Minnesota Earthquakes Since 1860
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Long
Prairie
1860-61 5.0
New
Prague
1860
4.7
St. Vincent 1880
3.6
Red Lake
1917
3.8
Staples
1917
4.3
Bowstring
1928
3.8
Detroit
Lakes
1939
3.9
Alexandria 1950
3.6
Pipestone* 1964
3.4
Morris
1975
4.6-4.8
Milaca*
1979
1.0
Evergreen
1979
3.1
Rush City
1979
0.1
Nisswa*
1979
1.0
Cottage
Grove
1981
3.6
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Walker
http://www.morris.umn.edu/earthquakes/articles/jan301990.html
1982
2.0
*Earthquakes that were recorded instrumentally.
All other earthquakes and associated magnitudes
based solely on local reports.
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