Geophysics

Not the Fault of Compaction

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Science  07 Apr 2006:
Vol. 312, Issue 5770, pp. 21
DOI: 10.1126/science.312.5770.21c

Deltas represent a huge accumulation of sediment; large ones are often the sites of major cities, such as New Orleans. Several processes—compaction of the sediment, withdrawal of ground water and oil (which accelerates compaction), and sea level rise—lead to subsidence and inundation of the deltas and to associated problems for their cities. These are compounded when the supply of new sediment is interrupted, as is commonly the case.

On the other hand, subsidence can also be related to faulting induced within the huge sediment pile. By analyzing a large number of leveling benchmarks tied to modern Global Positioning System data, Dokka shows that subsidence over the past 50 years around New Orleans has been dominated by such a fault and not by sediment compaction driven by groundwater pumping as has been presumed. The fault has down-thrown a 200-km-wide block extending north of New Orleans out into the Gulf of Mexico. The size of the fault block has made it difficult to recognize in local benchmark surveys, which thus could not reveal the absolute rates. Subsidence attributed to faulting may have reached about 17 mm/year around 1970 and several millimeters per year recently, which is comparable to the presumed compaction rate. — BH

Geology 34, 281 (2006).

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