Active Faults

Nancye DawersThe way Nancye Dawers views things, the creation of the rich soil of the Mississippi River delta happened almost yesterday.

An associate professor of earth and environmental sciences, Dawers says that the “young” Mississippi River delta was formed in the “recent” past, that is, the last 10,000 years.

Dawers is a structural geologist who studies the evolution of faults in the earth’s subsurface. She takes the long view—the millions-of-years-old view.

Millions of years before the Mississippi River as we know it began building its present delta, the thick sequence of sediments that underlay it began to fracture, and faults formed in response to the accumulating load of sediment. One of these faults eventually resulted in the formation of Lake Pontchartrain. The lake, which isn’t a lake at all but an estuary where fresh and salt water mix, is, of course, crucial to the situation of the city of New Orleans. While Lake Pontchartrain itself is only a few thousand years old, its northern shore is controlled by an ancient fault system, part of which is known as the Baton Rouge fault.

Most of Dawers’ work has been in the western United States—California, Idaho, Montana and Utah—where seismically active faults have dramatically affected land formations.

She came to Tulane and Louisiana seven years ago, not expecting to find an active fault along the northern margin of the Gulf Coast. But then she heard of a 1944 Mississippi River Commission report that presented evidence of an active fault near Baton Rouge from the Amite River to the Pearl River.

Dawers became intrigued that the Baton Rouge fault had played a major role in the formation of the Lake Pontchartrain Basin.

Slips on the Baton Rouge fault results in subsidence of the area immediately south of it, namely the Pontchartrain Basin. The fault may still be active, Dawers has discovered, although the fault appears to be “aseismic.” Aseismic means it doesn’t produce large earthquakes but rather steady, slow slippage occurs in association with small earthquakes. Some of these earthquakes occasionally are large enough to be felt in the region.

Using high-resolution laser technology and other techniques, Dawers and graduate student Bobby Cosentino are mapping the various segments of the Baton Rouge fault system, trying to determine its long-term history. By measuring the slope of the fault’s topographic scarp—the step it forms in the landscape—they can discriminate between the likely constant, slow displacement of land versus displacement in sudden, large earthquakes.

With another graduate student, Emily Martin, who earned her master’s degree in 2006, Dawers also has explored a fault in southern Plaquemines Parish near Bastian Bay and the town of Empire. They know from early oil exploration data that a subsurface fault exists in the area. There also has been land loss in the area traced to the 1970s—a time of active oil and gas production. However, the pattern of displacement of marshland at this locality is more consistent with a fault than with fluid withdrawal.

The issue of faulting in coastal Louisiana is a controversial topic, says Dawers, because “if faulting is contributing to our subsidence problem, there aren’t any solutions to stopping that process.”

Thousands of years can go by where nothing happens. And then there might be an episode of fault slip. “We just don’t know,” says Dawers, as she continues to look into displacement patterns and slip rates. Like most geologists, she does not predict the future; she seeks to understand processes, both present and past.

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