NEW ORLEANS -- A new study raises another possible threat to rebuilding efforts in Louisiana: Active geologic faults are causing levees, flood walls, bridges, and homes to sink.
The study, published in the April edition of the Geological Society of America's Geology journal, charts a major fault it says runs through eastern New Orleans.
It also contends that the fault's downward movement ''set the stage for the devastation of Hurricane Katrina by lowering elevations of the land and surrounding levee defenses."
The study was the result of ongoing work by Roy Dokka, a Louisiana State University geologist, and the National Geodetic Survey to calculate land changes in south Louisiana using global positioning system base stations and tide gauges.
Dokka's theories on how natural tectonic fault movements cause subsidence, or land sinking or sliding, run counter to previous studies that have pointed to oil extraction and soil compaction as main reasons.
If faults are causing the sinking, some specialists say there is very little humans can do to offset the subsidence other than build higher levees.
But if oil drilling is the main culprit, then there is hope that a slowdown in onshore drilling will mean less subsidence.
Bob Morton, a geologist with the US Geological Survey, blames faults oil drilling for large amounts of subsidence. ''Can faulting be induced? Absolutely, and he [Dokka] omits that completely," Morton said.
Dokka, though, said oil was not drilled near the Michoud fault, so the sinking wasn't caused by ''the usual suspects. . . .It has to be something deep."
Louisiana's coast has lost about 1,900 square miles of wetlands -- an area roughly the size of Delaware -- since the 1930s.
The study said the slumping of the Michoud fault has caused up to about 73 percent of the subsidence in sections of eastern New Orleans.
Al Naomi, a top US Army Corps of Engineers project manager, said more attention is being paid to subsidence and fault studies.