Scientists are increasingly utilizing dense arrays of seismometers spread over wide areas, in order to study and measure waves that pass through the Earth. That data allows them to map earthquake faults and other deep underground features that may not be evident at the surface.
University of Washington at St. Louis researchers, for example, have used 10 years worth of data from the National Science Foundation's Earthscope, a network that includes thousands of instruments, to create vidid images of a massive, ancient scar deep under the surface of the Midwest.
The Midcontinent Rift, as it is known, is evidence of an event that occurred approximately 1.1 billion years ago, in which the land that would become today's North America nearly was ripped apart by stresses within the Earth. If that had occurred, Minnesota and Wisconsin today might be on opposite coasts of an ocean.
The rift stretches southward from Lake Superior for nearly 1,900 miles.
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Scientists first discovered evidence of the rift in the mid-20th century, when they noticed that gravity was stronger in some parts of the Midwest than in others. They used sensors in aircraft to detect and create the first maps of the rift.
WUSL was part of a consortium of universities which installed 83 of these stations along and across the rift in 2011, creating a dense array called SPREE, which is part of Earthscope.
Shen, whom a WUSL press release describes as the research team's "data wizard," utilized a form of statistical analysis to combine many different types of seismic data, and to produce much clearer images of the scar under the surface than one type of data alone would produce, As this 2013 Nature article details, the rift is buried under deep sediments, which makes it challenging to study. Scientists have yet to find an answer for why North America didn't split apart.
Image credit: USGS via Wustl.edu