SHERIDAN COUNTY CONSERVATION DISTRICT

 

Did You Feel It ?

What a Way to Start the New Year

A minor earthquake struck northwest North Dakota and Northeast Montana in the early hours of Saturday, January 3, 2009.

According to the Montana Bureau of Mines and Geology at Montana Tech of the University of Montana in Butte, a 1.5 magnitude earthquake was recorded 11.3 miles ESE of Dagmar at 6:53 AM MST on Saturday, January 3, 2009.

The quake was minor and virtually unnoticeable, but was recorded at several different seismograph locations, including three in Sheridan County located near Dagmar, Westby, and Redstone. The Dagmar station was installed in 2004. Westby and Redstone were installed in September of 2008.

According to Michael Stickney, Director at the Earthquake Studies Office at the Montana Bureau of Mines and Geology in Butte “We feel we have a reasonably good epicenter location. The quake was tiny, with a magnitude of 1.5, but large enough to be recorded on the ANSS station at Dagmar, MT and 10 stations of the EarthScope Transportable Array. I would not expect that anybody in the epicentral area would notice such a small earthquake”. The epicenter was located at Latitude 48.537°N, Longitude 103.946°E at a depth of 13.4 kilometers, or 8.3 miles, beneath the surface. Source of earthquake location was determined by MBMG - Montana Bureau of Mines and Geology.

The EarthScope Transportable Array is a continental-scale seismic observatory designed to provide a foundation for the study of the continental deep Earth structure over a wide range of scales. The array is being constructed, operated, and maintained as a collaborative effort with UNAVCO, the Incorporated Research Institutions for Seismology (IRIS), and Stanford University.

The epicenter was first thought to be located along the Brockton-Froid fault zone, but was later pinpointed to a location in North Dakota. The Brockton-Froid fault zone is one of the major basement fault systems in the center of the continent, and has been interpreted by field mapping efforts of the U.S. Geological Survey as a northeast-southwest (N55E) trending fault zone more than 50 km (30 mi) in length.

It is believed an earthquake on June 24, 1943 was most likely centered along the Brockton-Froid fault zone. The estimated magnitude of the 1943 earthquake is about 4.5. The quake caused damage in Froid and elsewhere around the area. Local newspapers reported damage to chimneys and that a "well-constructed granary cracked so severely that wheat spilled out" at Froid. Plaster and chimneys also cracked at Homestead, Redstone and Reserve. Several other earthquakes have been associated with the fault, including 3.5 to 4.0 quakes in the 80s and 90s.

Stickney added, “I guess this earthquake goes to show that you never know what you might see when a seismic network moves into your neighborhood”.