Earthquakes along Mississippi could be long-after shocks

By Dan Vergano, USA TODAY

Mid-continent earthquakes along the Mississippi and elsewhere might just be long-lived aftershocks of big quakes, not fresh events, geophysicists suggest in a new study.

Through centuries, mid-continent faults have proven capable of unleashing horrible quakes, such as 2008's magnitude-7.9 earthquake in China, which killed at least 69,195 people. But the study, published in the journal Nature, suggests such quakes spawn aftershocks that linger for centuries. That differs from earthquakes on continental edge faults, such as California's San Andreas, which release aftershocks within days.

"Every large earthquake in the continental center comes as a surprise," says study co-author Mian Liu of the University of Missouri-Columbia. "We think we need to step back and reconsider the physics at these places."

In the 20th century, geophysicists demonstrated that continental crusts move over the Earth's surface over hundreds of millions of years, a phenomenon called continental drift. Where continents clash or collide with the ocean floor, earthquakes and volcanoes result. Mid-continental earthquakes, however, awaken more ancient, seemingly quiet faults left over from long-ended continental collisions.

In the study, Mian and Seth Stein of Northwestern University in Evanston, Ill., show that earthquakes in continental interiors, such as the Mississippi's New Madrid seismic zone hit by 1811 and 1812 major quakes, unleash aftershocks centuries to millennia later. Rather than pointing to risks of future major quakes on those spots as they would on the edge of a continent, these events are simply the slow settling of continental crust.

Mian says the risk of new major quakes "is probably lower than people suppose" along the New Madrid fault and around Charleston, S.C., hit by a major quake in 1886. U.S. Geological Survey maps show both areas as susceptible to major quakes, which affects building codes.

However, "suggesting these are just aftershocks and therefore (the New Madrid fault) is dead doesn't follow," says Susan Hough of the U.S. Geological Survey office in Pasadena, Calif. Other major quakes have hit the fault, which stretches from Indiana to Mississippi, around 1450 and 900, as well as perhaps as far back as 2000 B.C. "We have a seismic zone that produces earthquake clusters. The odds that it has shut off are pretty low."

In a commentary with the study, the USGS's Tom Parsons compares forecasting mid-continent quakes to "predicting a full year's weather based on watching one week in January."