The red squares mark the locations of SPREE seismic stations. The north south row follows the rift. The Dodge Center station is located on the lower row crossing the rift and is to the right of the intersection. This is a map of the gravity anomaly across the area. Artwork supplied by Professor Doug Wiens.
The fence encloses the Flexible Array seismic station near on Dennis McNeilus' property. The solar panels power the seismometer and data recorder installed in a shallow underground vault. Photo by Larry Dobson
By Larry Dobson
There is a seismograph just outside Dodge Center being used to study Earth's structure by making daily recordings of earthquakes. A few days after it was installed, on April 29, 2011 it recorded a magnitude 2.5 Minnesota earthquake at Alexandria. However, its purpose is not to study Minnesota earthquakes.
A billion years or so ago, the area where we live on the North American continent began to split up and drift apart forming a "rift" that arcs from northeastern Kansas through Nebraska, Iowa, Minnesota and Wisconsin to the north shore of Lake Superioraalong Ontario, Canada then down through Michigan. Geoscientists say the rift should have continued to drift apart to form an ocean but after 15-20 million years it stopped and became what is now called a failed rift and named the Midcontinent Rift (MR). No one knows why rifts fail, so scientists have set up a series of Flexible Array seismic stations along and across the Midcontinent Rift to learn more about them, and one of those stations is located in Dennis McNeilus' field north of Dodge Center.
Professor Doug Wiens, Chair of the Dept. of Earth and Planetary Sciences at Washington University in St. Louis, Missouri is one of the geoscientists working on the SPREE Project, as the study of the Midcontinent Rift is called. It stands for Superior Province Rifting Earthscope Experiment. Wiens set up the seismic station in McNeilus' field and either comes or sends someone periodically to collect the data the seismic station has recorded.
Since no one knows why some rifts succeed and others fail, Wiens said data recorded by the seismometers will be used to create images of the rift's deep subsurface structures, something like taking an ultrasound of the Earth. The seismometers will record seismic waves from earthquakes that occur locally, regionally and throughout the world which scientists will use to produce high-resolution images of the Earth's interior, particularly along the Midcontinent Rift. From those images they hope to learn how the rift started, grew and then failed.
Wiens said many rifts break continents apart to form new oceans, as happened when Europe and North America rifted apart to form the Atlantic Ocean, but here, despite its size, the Midcontinent Rift failed to break apart the North American continent.
Some geologists see the Midcontinent Rift as being analogous to the East African, Red Sea, Gulf of Aden rift system and speculate that there may be large reservoirs of oil and natural gas waiting to be tapped. Rifting, successful and failed, shapes continents and provides conditions for deposition of hydrocarbons and other mineral resources. There have been live oil seeps from subsurface fractures along the rift in Michigan. Exploratory wells along the rift in Wisconsin, Minnesota and Iowa have so far come up dry.
Wiens said the SPREE Project is not looking for hydrocarbons but that the data obtained may be useful to those who do so in the future.
SPREE is a collaboration between the University of Minnesota, Northwestern University and Washington University in the U.S. and the University of Manitoba and the University of Quebec in Canada. It is funded by a grant from the National Science Foundation through Earthscope (www.earthscope.org). Eighty-three seismic stations were installed, 16 in Canada and 67 in the U.S. There are two lines of stations in the U.S., one that follows along the rift and one that crosses the rift. The Dodge Center station is located on the line that crosses the rift and not far from where it intersects with the line along the rift.
The seismic stations are about 10' by 10 ' and are fenced in for protection. The seismometer is buried underground in a shallow vault on a concrete base. The seismometer and data recorder are powered by a solar panel. Wiens said he or one of his grad students comes to Dodge Center twice a year to collect the data recorded by the station.