In the wake of Hurricane Katrina, it's become widely known that New Orleans has been slowly sinking. Geologists have blamed oil drilling, groundwater pumping, and young, soft sediments for much of the region's subsidence, but a new study implicates another culprit. The deep shifting of tectonic plates may be causing the land to sink faster than the shallower manipulations of humans. That could mean more drastic measures need to be taken to protect New Orleans from another storm.
Geologist Roy Dokka of Louisiana State University in Baton Rouge focused his study on an area of the city known as the Michoud, on the southeastern shore of Lake Pontchartrain. The Michoud has no oil, gas, or water extraction, which causes sediments to compact, but is underlain by a 7-km-deep fault. It also has some of the highest subsidence rates in the south-central United States. Could nature be to blame?
Dokka sought the answer by taking advantage of 50 years of data. The National Oceanic and Atmospheric Administration had conducted multiple surveys of the Michoud region, beginning in 1955. One key benchmark was a 2000-meter-deep solid steel well. Because human activities such as drilling and the natural settling of soil occur within the first 2000 meters of Earth's surface, the well would stay at the same elevation unless movements were occurring underneath it--where the Michoud fault lies.
The study area was sinking an average of 16.9 millimeters per year between 1969 and '71 and 7.1 millimeters per year between 1971 and '77, Dokka reports in the April issue of Geology. Using his deep benchmark, Dokka calculated that tectonic activity was responsible for 73% and 50% of the subsidence in those two periods; the rest was likely due to sediments compressing and recently deposited soils draining. This indicates "that there's a big chunk of subsidence occurring in a place that cannot be explained by other activities," says Dokka. Merely stopping water extraction and oil drilling off the coast might not help protect New Orleans from being inundated by future hurricanes, he says.
Not all geologists are convinced. Orrin Pilkey of Duke University in Durham, North Carolina, admits that Dokka "makes the case for including tectonics" among the factors contributing to the sinking of New Orleans. But both Pilkey and Robert Morton of the U.S. Geological Survey in St. Petersburg, Florida, caution that Dokka's results don't mean that the oil drilling and other human actions aren't having an effect. "I would have major problems if people were coming away with broad conclusions about what's happening from this limited area," says Morton.