ABSTRACT

Active geologic faults in Lake Pontchartrain have been previously described in 1991. These faults have apparent surface expression which strongly suggests that at least some of these faults are active. The en-echelon fault system in northern Lake Pontchartrain are the southeastward continuation of the active Baton Rouge - Denham Springs faults system in the Baton Rouge area. New documentation of faulting includes: 1) Global Positioning System was used to locate bridge offsets, 2) acquisition of US Geological Survey high resolution seismic, 3) more complete mapping of faults in Lake Pontchartrain and, 4) new quantification of Railroad and bridge offset at fault traces.

Three U. S. Geological Survey high-resolution seismic profiles demonstrate two geologic faults with offset within the Holocene within three meters (10 ft) of the Lake bottom in the eastern portion of the Lake. In contrast, one high resolution seismic profile positioned parallel to the Causeway does not detect a fault at the offset in the Causeway bridge. The fault along the Causeway is either inactive, or the offset is too small to be detected by the high resolution seismic.

Recent observations suggest possibly higher rates of fault movement than previously recognized in Lake Pontchartrain. In particular, the Norfolk-Southern Railroad bridge has shown 8-10 cm (3-4 inches) offset at the southern fault position where the bridge was re-built in 1986 and 1987. The northern fault trace shows a similar new offset of the Southern railroad bridge. This suggests a rate of fault displacement two-three times greater than suggested in 1991 by previous observations. However the greater movement rate is consistent with other reported rates of modern fault movement in the Houston, Texas area of the Gulf Coast.