Earthquake activity in the United States is not limited to the coastlines, and a study released today indicates that the Midwest region of the United States is due.
At least parts of seven states comprise an area called the New Madrid Seismic Zone, which runs 150 miles within Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri and Tennessee. The fault also crosses the Mississippi River in three places and the Ohio River twice. According to the U.S. Geological Survey, these areas are at a high level of risk for earthquakes, even though scientists are still trying to figure out exactly why.
Because this area of the country is located centrally on the North American plate, it is difficult to imagine earthquakes occurring there. Scientists, however, have gathered evidence that proves that over the past 4,500 years, major earthquakes measured at 7-8 magnitudes have indeed occurred in the Midwest and that the region is due for another.
As recently as 1811 and 1812, the New Madrid Zone was the location of a series of very strong earthquakes measuring between 7.5 and 7.7 magnitudes. During the time between Dec. 16, 1811, and Feb. 7 of the next year, three earthquakes and an equally strong aftershock occurred in Arkansas, Missouri, and Tennessee.
Reports from the time indicate that church bells rang in Boston and Charleston, South Carolina, and that the Mississippi River ran backwards during the quakes. Fissures as long as five miles swallowed residents, and one person was killed as far away as Louisville, Kentucky. Damage to structures and devastating loss of life was limited due to the sparse population.
Thousands of what were believed to be aftershocks were felt from the earthquakes, and quakes occurred in 1843 and 1895 that are estimated to have been around 6.3 to 6.6 magnitudes. Theories to date have hypothesized that all of the seismic activity that occurred was caused by the original earthquake of 1811.
The USGS scientists re-examined the long-ago quakes using the Omori decay law, which predicts the rate at which aftershocks decrease as time passes. Using this law, they were able to disprove the theory that the seismic activity in the New Madrid Seismic Zone was caused by aftershocks from the original earthquakes of 1811 and 1812. In addition, the scientists report that if the area had been experiencing aftershocks, the seismic strain would be diminishing and the risk for earthquakes would be lessening, but the exact opposite appears to be occurring and the risk in the New Madrid Zone is still high.
Scientist Susan Hough of the USGS, one of the authors of the study released today, told NPR that her research shows that something is “alive and kicking” beneath the earth’s crust in that area.
While the risk itself is high, scientists estimate that the chance that another earthquake occurs in the region within the next 50 years is only 7-10 percent. Hough expects that the area might see an earthquake measuring around 7.0 one time every 500 years. The possibility for a 6.0-magnitude quake averages to be about one time every 50 years, with the last occurring in 1895. Those numbers suggest that the Midwest region of the U.S. is indeed due for another earthquake. Detractors to the conclusions reported today question the results due to the fact that the methods used were developed to study faults along plate boundaries, but not when they occur in the middle of a plate, such as in the New Madrid Seismic Zone.
By Jennifer Pfalz