THE MAGAZINE

The Question of a Heartland Quake

By Joseph Straw

While rare, earthquakes in the Midwest pose a greater danger than earthquakes in the West because of soil types and building codes.

Earthquakes aren’t usually associated with the Mississippi, but along with the constant threat of weather-related disaster, the states along the central Mississippi River do indeed face the risk of a tectonic shift. The New Madrid Fault, or Seismic Zone as it’s referred to by scientists, stretches in the shape of a crooked “T” from northwest Arkansas into the “Bootheel” tip of Southeast Missouri and then across the Mississippi into Northwest Tennessee.

While seismic activity is most often associated with places where the plates of the earth’s crust meet, New Madrid (pronounced MAD-rid, after the Missouri city) is rare in that it is an intraplate fault—at the center of the North American Plate.

As with nearly all geological phenomena, the seismic zone’s nature is not fully understood, even by those who have spent their entire careers studying it. Yet its potential for severe earthquakes is a fact of history. An 8.0 magnitude earthquake in 1812 is the strongest event ever recorded in the 48 contiguous states; ten times stronger than the October 1989 Loma Prieta quake that killed 68 in the San Francisco Bay Area.

A magnitude 6 quake, 100 times less severe than 1812’s, last hit the region in 1895.

While rarer than those along the West Coast, severe earthquakes in the Midwest pose a broader risk, experts say, for a few reasons. First, few buildings, even new ones, are built to withstand tremors like those along the Pacific Rim. Second, the soft, silty soil of the Mississippi basin is more prone to “liquefaction” than that in the West. Third, bedrock in the East carries seismic waves farther than bedrock west of the Rocky Mountains. The 1812 quake, for example, rang church bells in Boston.

In recent decades, miniscule tremors have occurred frequently, but most of them have been discernable only to seismographs. A barely palpable, 2.3 quake struck near Cairo, Illinois, last November. 

Aware of the risk, eight states in the region formed the Central United States Earthquake Consortium (CUSEC) in 1984 to coordinate preparedness for a major quake—which is considered magnitude 5.5 or greater. Then, last year the Federal Emergency Management Agency (FEMA), reminded by Hurricane Katrina that worst-case scenarios do materialize, launched the New Madrid Seismic Zone (NMSZ) Initiative to assess preparedness and coordinate planning.

Amid these efforts one seismologist from Illinois’ Northwestern University, Seth Stein, has stirred controversy by predicting, based on research, that there is no chance of a major earthquake in the area.

Stein bases his prediction on research of land movement in the New Madrid region, which scientists now conduct using global positioning system receivers. His finding: The earth around the New Madrid Fault is not moving, and if portions of the earth’s crust are stationary relative to one another, tension does not build along faults, and earthquakes do not occur. Stein explains that intraplate faults produce major earthquakes like that of 1812 as a result of “episodic migration,” then fall silent.

David Hoffman, a professor and research engineer at the University of Missouri-Rolla, stands with most U.S. geologists who see a constant, if not imminent, threat of a major quake in the area. He points to the region’s sedimentary record, which suggests prior major quakes in the years 900 and 1450.

Hoffman concedes that predicting seismic activity is highly speculative. “But the vast majority of people believe that what you’ve had in the past is what you’re going to have in the future, which has been a repeated series of large earthquakes,” he says.

Officially, the U.S. Geological Survey (USGS) sees a 7 to 10 percent chance of an 8.0 quake over the next 50 years, and a 25 to 40 percent chance of a 6.0 quake, according to recent congressional testimony by USGS scientist David Applegate.

The region’s state emergency managers—who form CUSEC’s board of directors—don’t concern themselves with the ongoing debate.

 “One of the things the scientists talk about is the probability or likelihood of an earthquake within ‘x’ number of years, but usually a seismologist will end by saying, ‘but it could happen tomorrow.’” says David Maxwell, director of the Arkansas Department of Emergency Management. “Well, morally we have a responsibility to prepare, and let the scientists argue about the probability.”

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