Activity In New Madrid Seismic Zone Suggests A Midwest Earthquake Could Happen

By Ajit Jha on January 26, 2014 2:26 PM EST

New Madrid Seismic Zone
Scientists have debated whether the New Madrid Seismic Zone (shaded area) is still active. And with many recent tremors (indicated by the dots), authors of a new study say there's still a chance for a major earthquake. (Photo: USGS)

The New Madrid Seismic Zone in America's Midwest experiences over 200 earthquakes every year. In fact, huge earthquakes that occurred over 200 years ago caused the Mississippi river to turn its course backwards. As scientists continue to debate whether these tiny quakes could mean the fault is old and dying or a precursor to another massive quake, a new study finds that minor shocks along the Reelfoot Fault are happening more often than expected.

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Reports of the dying seismic zone could have been premature. "It's not going to go off anytime soon, but we do have evidence that more stress is being built up now," said Morgan Page, lead study author and a geophysicist with the U.S. Geological Survey (USGS) in Pasadena, Calif., according to LiveScience. "Eventually, that energy will have to be released in a large earthquake."   

Hidden under the Mississippi River are the faults that comprise the New Madrid Seismic Zone. The Reelfoot Fault, which is located along the borders of Arkansas, Missouri and Tennessee, was the site of four huge earthquakes and several aftershocks in late 1811 and 1812. Each of these shock waves had an estimated magnitude between 7 and 8 - strong enough to shake eastern North America, and destroy the town of New Madrid, Mo.

There is now a low probability of earthquakes in the New Madrid Seismic Zone - once every 10,000 years or more - but the study is a warning against believing that the zone has settled down. It claims that the ongoing low-intensity shock waves in the New Madrid Seismic Zone are a buildup of seismic energy on the faults. "Even though we can't predict earthquakes, we can predict the rates of aftershocks over time," Page said, according to LiveScience. Called Omori's Law, it states that the smaller shocks following a big earthquake decrease over time. However, due to their ongoing nature, these shocks don't seem to be aftershocks.  

According to the researchers, if the earthquakes over the past 200 years were aftershocks, then there should have been over 130 magnitude-6 earthquakes, each bigger than the Virginia earthquake of 2011 - and that's not the case. These earthquakes are unrelated, according to Page, who said that "200 years is too long for an aftershock sequence. Instead, we think stress is being built up now."

Meanwhile, scientists who claim they are not precursors to a large-scale earthquake say that they could still be aftershocks, arguing that they could just behave unpredictably. "We would expect faults within plates to have long aftershock sequences," Seth Stein, a seismologist at Northwestern University in Illinois who was not involved in the study told LiveScience. While Page agrees that seismic zones behave differently due to structural differences, she claims that her modeling accounted for this difference. "Even though aftershocks in intraplate regions do go on longer, they still follow Omori's Law," Page said.

The modeling approach used by Page and her colleague Susan Hough, according to some scientists, is inapplicable to the middle of tectonic plates, as opposed to their edges. "I think one thing is clear in that they have shown the central U.S. behaves differently than California in terms of seismicity and the expected behavior of aftershocks," Charles Langston, director of the Center for Earthquake Research and Information at the University of Memphis in Tennessee, told LiveScience. "Current New Madrid seismicity has other characteristics that make it unusual and very different from California seismicity."

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