Star Tribune
June 23, 1994

By Lewis Cope; Staff Writer

Relax a little.

In this season of thunderstorms and tornadoes, here are other types of quick-striking natural disasters that you won't have to fret so much about:

Earthquakes.

Don't let this month's little shake from Bolivia alarm you.

"Minnesota's earthquakes risk is about as low as it gets the nation," said Val Chandler of the Minnesota Geological Survey.

The only area that's less prone to generating earthquakes is along the Gulf of Mexico coast, where soft rock and clay formations are difficult to shake, he said.

Earthquakes that occur in neighboring states or even farther are felt by some Minnesotans. The quake in Bolivia that shook the ground a bit in parts of the Twin Cities and elsewhere on June 8 was extremely deep and very powerful.

But no quakes -- either home-grown or felt from a distance -- have caused any known deaths, severe injuries or major damage in Minnesota, Chandler said. Minnesota didn't have any reports of injuries or damage from the Bolivia-generated tremor.

The Pacific-rim risk: The Earth's crust is made of gigantic plates that form the ocean basins and continents. The sliding together of the Pacific Plate and the North American Plate has created the faults (cracks) and pressures in the California rock formation, making that area prone to quakes.

Similar plate crashes make areas all around the Pacific Rim -- from Bolivia to Japan -- vulnerable to quakes. Minnesota's mid-continent location puts it at a very safe distance from this action, Chandler noted.

Minnesota: There are some cracks in all major rock formations. But Minnesota's bedrock is relatively strong, and there are no major forces pushing on existing cracks to make for either big or frequent quakes, he said.

Minnesota home-grown quake history:

- Only 19 earthquakes have been reported in the state since the earliest recorded one, which occurred near Long Prairie in about 1860.

- The only quake that was centered in the Twin Cities area was in Cottage Grove in 1981. While some people felt the shaking, there were no reports of any injuries or damage.

- The most powerful Minnesota quake of this century was in 1975, centered near Morris in western Minnesota. It rattled dishes and windows, cracked some plaster and caused some other minor damage. One person fell out of a haystack and sustained minor injuries. This quake measured 4.6 to 4.8 on the Richter scale. Last century's Long Prairie quake may have reached 5.0.

Richter comparisons: The 7.1 quake in the San Francisco Bay area in October 1989 killed 62 people, injured 3,750, and incurred $6 billion in damages. On the Richter scale, a 7.1 quake radiates about 1,500 times the energy of a 5.0 quake.

The Bolivian quake was even more powerful - 8.2 on the Richter scale. Its great depth (about 400 miles below the surface) helped minimize damage in Bolivia. This depth also allowed it to be felt more than 4,500 miles away in the Twin Cities and even into Canada.

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