Are shifts in Earth's crust causing New Orleans to sink?
Hurricane Katrina's devastating strike on New Orleans last fall highlighted shortcomings in the city's levee system. It also focused attention another long-term problem: The city and the region around it are sinking.
New research suggests, however, that at least for nearby Michoud, La., the dominant driver pulling the region under may not be among the usual suspects: oil extraction, pumping groundwater to the surface, or diverting the Mississippi for navigation.
Instead, the King of Slump may be a deep fault that cuts across southeastern Louisiana and under Michoud. During the 1970s, the fault appears to have contributed from 50 to 73 percent of the subsidence in this section of Orleans parish, depending on the time period measured. If sustained over a century, that would equate to as much as a six-foot sea-level rise, independent of any increase tied to global warming.
"Something dynamic is going on down there," says Roy Dokka, who heads the Center for GeoInformatics at Louisiana State University in Baton Rouge, La. "It doesn't occur everywhere," but it certainly appears to be affecting Orleans parish, he adds.
If these results hold up, they would imply that to build new levees properly, engineers will have to take into account the effects of further slumping along the fault - data hard to come by because the fault is so deep and difficult to study.
The work is controversial. It builds on a study Dr. Dokka and Kurt Shinkle of the National Oceanic and Atmospheric Administration's National Geodetic Survey (NGS) completed in 2004 for NOAA. That study drew on some 2,700 measuring points around southern Louisiana to measure subsidence rates. It yielded far higher sinking rates than other scientists had calculated.
Undaunted, Dokka says he suspected that tectonic forces might account for the difference. So he analyzed measurements from a smaller group of these "benchmarks" that straddle the Michoud Fault. Several benchmarks are associated with water-well casings that reach as deep as 1.2 miles - far below sediment layers that would be affected by removing ground- water in the region or by compaction, Dokka says.
He found that the deepest layer contributed far more to subsidence during the study periods than did intermediate and upper layers of sediment. He attributes the high rates to the fault, which appears to release stress in a creeping "earthquake," rather than in a sudden snap. The results appear in the April edition of Geology, a journal of the Geological Society of America.
To Arthur Berman, a petroleum geologist in Houston who has tracked the issue, Dokka's work is solid. His only quibble: The study is using the best available data, but they're from 1988. So they don't reflect what is going on today. Updating the 1988 NGS survey would cost millions of dollars.
Still, he says, he's not surprised by the results. "This is solid geology," he says. "When there are changes, the first thing I would look at is the basin itself."
The next step, Dokka says, is to apply GPS satellite-navigation technology to the problem. As they have elsewhere, he says, GPS receivers should be able to track changes in height and any lateral movement in the land in great detail. This could help apportion the causes of subsidence among the various factors scientists have identified and cover a wider area.
