The 1975 Morris earthquake occurred in Morris, Minnesota

Morris, Minnesota

As of the census of 2000, there were 5,068 people, 1,929 households, and 985 families residing in the town. The population density was 1,183.2 people per square mile . There were 2,067 housing units at an average density of 482.6 per square mile...

, on July 9, 1975 at 14:54:15 UTC, or 9:54 a.m. The strongest instrumentally recorded rupture in the history of the state

Minnesota

Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

, it registered at magnitude 4.6 on the Richter scale. It was the first earthquake

Earthquake

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

 to be recorded on any seismic instrument in the state since 1917. Tremors were felt over much of Minnesota, northern Iowa, and the eastern Dakotas.

Setting

Minnesota is not a very tectonically active state; however, there is at least one fault zone in it, the Great Lakes Tectonic Zone

Great Lakes tectonic zone

During the Late Archean Eon the Algoman orogeny added landmass to the Superior province by volcanic activity and continental collision along a boundary that stretches from present-day South Dakota, U.S., into the Sudbury, Ontario, Canada, Lake Huron region....

, stretching from Big Stone County and Traverse County

Traverse County, Minnesota

As of the census of 2000, there were 4,134 people, 1,717 households, and 1,129 families residing in the county. The population density was 7 people per square mile . There were 2,199 housing units at an average density of 4 per square mile...

 to Duluth

Duluth, Minnesota

Duluth is a port city in the U.S. state of Minnesota and is the county seat of Saint Louis County. The fourth largest city in Minnesota, Duluth had a total population of 86,265 in the 2010 census. Duluth is also the second largest city that is located on Lake Superior after Thunder Bay, Ontario,...

. Seventeen earthquakes have occurred along the fault, the two largest being the Morris quake and the Staples event of 1917. However, plotting of the earthquakes suggests that there may be another hidden fault in the state somewhat adjacent to it.

Damage

The earthquake caused moderate damage, and was defined as intensity VI on the Mercalli scale. Minor damage to walls and basement foundations was reported around the epicenter

Epicenter

The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...

 in Stevens County

Stevens County, Minnesota

As of the census of 2000, there were 10,053 people, 3,751 households, and 2,366 families residing in the county. The population density was 18 people per square mile . There were 4,074 housing units at an average density of 7 per square mile...

. Damage consisted of cracked plaster, falling items and pictures being knocked off walls. The quake also cracked the foundations of two homes on East Fifth Street in Morris. Many reports included extremely loud "bangs" and "explosions". These sounds lead to one man thinking that a nearby gas plant had exploded. If the earthquake had occurred elsewhere, however, it may have caused a larger amount of damage.