GEODETIC EVIDENCE OF ACTIVE FAULTING IN SOUTHWEST LOUISIANA

Benchmark velocities derived from analysis of first-order geodetic leveling lines from the National Geodetic Survey and tide gauge data from the National Oceanic and Atmospheric Administration have provided new insight into the activity of known faults in southwest Louisiana. Because of an emerging consensus that faulting could play a major role in the subsidence and subsequent land loss of coastal Louisiana, a better understanding of fault motion is essential to the success of coastal restoration. The majority of faults in south Louisiana (1) strike generally east to west, (2) are relatively steep near the surface (~ 70 degrees) but become shallower at depth, (3) and are normal growth faults, where the hanging block is downthrown and faulting takes place contemporaneously with sediment deposition. Consequently, faults in coastal Louisiana often show little or no surface expression and therefore have been mapped primarily with subsurface data by petroleum geologists working with seismic imagery and oil well boring logs.

A comparison of benchmark elevations from the 1960’s and 1980’s has shown that slip rates along normal faults in southwest Louisiana range from about 2 mm/yr to as much as 6 mm/yr. These rates can be updated with respect to current elevations by taking short-term GPS observations on benchmarks adjacent to fault traces. It is concluded that episodic fault movement has occurred in Louisiana throughout recent geologic history and using historical leveling data in concert with present day GPS observations should provide a better understanding of the nature of this movement.