Like a smaller landslide on the side of a hill, the huge Southern Louisiana landslide has a "headwall" where the slide is breaking away and a "toe" out in the Gulf where the debris from the slide is piling up, Dokka explained. The only difference from a traditional landslide is that this one is far, far larger and it's buried under lots of wet sediments, so it requires very accurate survey measurements to detect it.
Dokka obtained that data by re-examining decades of geodetic survey data from the National Oceanic and Atmospheric Administration (NOAA) in the area and calibrated it with the more stable Global Positioning System.
With that he was able to detect movements in the ground that were not visible to surveyors who used less reliable benchmarks like the more changeable sea level. He also used data from some very deep wells to see how much the land has been compacting from the removal of water and petroleum â€“ a common cause of subsidence.
His conclusion was that while there are certainly local subsidence effects from the over-pumping of groundwater and the river levees keeping sediments from reaching wetlands, these things are sitting atop a much larger block of earth that's been sliding into the Gulf of Mexico for eons. Dokka published a paper on his work in the April issue of Geology.
"Here it's subsidence that's occurring deep in the Earth," said Dokka.
As a result, there are faults along which the motion is taking place. One such fault, the Michoud Fault, runs right through New Orleans and is essentially the place where the sliding section of earth is breaking away.
The huge downward slide of the Mississippi Delta is pretty much what current geological theories would predict, said geologist Art Berman, who writes about such matters for the Houston Geological Society and works as a petroleum geologist in the Gulf of Mexico.
"We're at the margin of an active basin," said Berman. "There's 50,000 feet of sediment in the Gulf of Mexico that's pretty young."
All that sediment has weighed down the crust of the Earth there and caused it to sink.
"This is probably pretty common in areas like this worldwide," Dokka said.