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Earthquake Summary

Tectonic Summary

EARTHQUAKES IN THE STABLE CONTINENTAL REGION

Most of North America east of the Rocky Mountains has infrequent earthquakes. Here and there earthquakes are more numerous, for example in the New Madrid seismic zone centered on southeastern Missouri, in the Charlevoix-Kamouraska seismic zone of eastern Quebec, in New England, in the New York - Philadelphia - Wilmington urban corridor, and elsewhere. However, most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake. The earthquakes that do occur strike anywhere at irregular intervals.

Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

FAULTS

Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most of the region's bedrock was formed as several generations of mountains rose and were eroded down again over the last billion or so years.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. All parts of this vast region are far from the nearest plate boundaries, which, for the U.S., are to the east in the center of the Atlantic Ocean, to the south in the Caribbean Sea, and to the west in California and offshore from Washington and Oregon. The region is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even most of the known faults are poorly located at earthquake depths. Accordingly, few earthquakes east of the Rockies can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. In most areas east of the Rockies, the best guide to earthquake hazards is the earthquakes themselves.

Minnesota

Earthquake History

The first record of an earthquake in Minnesota was in 1860. Although the exact date is unknown, the shock was fairly strong in the central part of the State. Very little information is available about this earthquake.

On September 3, 1917, another earthquake shook the central part of Minnesota. Intensity VI (Modified Mercalli scale) effects were noted at Staples. The shock was also felt at Brainerd, about 30 miles east of Staples.

Several tremors located outside of Minnesota have been felt within the State's borders.

On November 15, 1877, two earthquakes 45 minutes apart occurred in eastern Nebraska. The shocks caused damage at North Platte and Columbus, Nebraska and at Sioux City, Iowa. The felt zone encompassed an elliptical area roughly 600 by 300 miles, including the southwestern part of Minnesota.

A strong earthquake centered in Illinois occurred on May 26, 1909, affecting an area of approximately 500,000 square miles, includings parts of Minnesota. Intensity VII effects were noted over a considerable area from Bloomington, Illinois to Platteville, Wisconsin. Many chimneys fell at Aurora, Illinois. Although details ae lacking, this shock was probably felt at intensity IV or V in southeastern Minnesota.

A strong earthquake on February 28, 1925, centered in the St. Lawrence River region near La Malbaie, Quebec, Canada, was felt widely in the Northeastern United States. The shock was lightly felt at Minneapolis.

Ten years later, on November 1, 1935, another strong earthquake occurred near Timiskaming, Canada and was felt over an area of the United States estimated at one million square miles. This tremor was also lightly felt at Minneapolis.

A recent earthquake to affect Minnesota occurred on November 9, 1968. Centered in south-central Illinois, the earthquake was felt over approximately 580,000 square miles of the Central United States. Minor damage was reported at Chicago, Illinois; Evansville, Indiana; and St. Louis, Missouri, as well as from a number of cities and towns in the epicentral area. Intensity I-IV effects were noted in Minnesota at Austin, Glencoe, Mankato, Minneapolis, Rochester, and St. Paul.

Abridged from Earthquake Information Bulletin, Volume 6, Number 1, January - February 1974, by Carl A. von Hake.

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