Aug. 29, 2007 — Two years after Katrina, the planet's message to southern Louisiana is loud and clear: The land is sinking there and nothing can stop it.
Or that's the message from geologists, anyway. The latest science on the sinking comes from a study in which researchers modeled how the Earth's rigid crust, or lithosphere, there is responding to the weight of Mississippi River sediments.
"Every geologist knows if you put a load on top of the Earth, it's going to cause the lithosphere to bend," said geologist Roy Dokka of Louisiana State University. Dokka, along with Caltech's Erik Ivins and Ronald Blom, published their results in the August issue of Geophysical Research Letters.
Their models were designed to see whether the sinkage, apparent in actual survey and extensive GPS data, can be explained by deep geological processes, rather than short-term, local causes — such as groundwater pumping, soil compaction or vertical movements along relatively shallow local faults.
"The GPS measurements are in reasonably good agreement with our model predictions, both in magnitude and spatial pattern," reported Dokka and his colleagues. Both data sets show the coast is sinking into the Gulf — a process known as subsidence — at a rate of about 1 to 8 millimeters (1/25 to 1/3 of an inch) per year.
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The subsidence of the Mississippi Delta has never been a surprise to most geologists. It is, in fact, the traditional way large river deltas are thought to behave all over the world. It's the reason why, for instance, there are ancient sunken ruins of cities found in the Nile Delta region today.
But figuring out exactly what's causing the subsidence is critical in light of plans to rebuild New Orleans — a city which has always been sinking, though gradually, along with its levees. Even before Katrina, it was long understood by officials and scientists that it was only a matter of time before a storm would lead to widespread flooding in the city, which lies below sea level.
