Normal faults that offset Quaternary sediments fringing the Gulf of Mexico are best known in the vicinity of Houston, but have been recognized from east of Baton Rouge, Louisiana, to the Mexican border--a distance of nearly 1,100 km. Throughout this large area, scattered faults 1 to 20 km long are active--a term that is here limited to faults whose movements have damaged man-made structures. High-resolution shallow seismic lines across selected faults demonstrate that scarps mapped at the surface represent only the most recent displacements along faults that persist to depths in excess of several hundred meters and show evidence of continued Quaternary movement. Additional data support the general conclusion that observed scarps are the surface expressions of both Tertiar growth faults and faults associated with the intrusion of salt domes.

Current fault activity is probably related to both natural and man-induced factors. Topographic maps based on 1915-16 surveys provide direct proof that some faults had already displaced the land surface before large-scale fluid extraction had significantly altered the stress state within shallow subsurface sediments. This and additional evidence suggest that natural faulting of the land surface was characteristic of the Quaternary history of much of the Gulf Coast, and locally may be continuing. In general, however, natural rates of fault motion are probably so low as to be of little consequence to man. Damage resulting from current fault motion is more likely attributable to widespread extraction of subsurface fluids. Several observations suggest that most offset of the land surface n the heavily pumped Houston area has taken place only within the last few decades: (1) few scarps are evident on early topographic maps; (2) faults are more visible on recent (1970s) aerial photographs than on photographs of comparable scale and quality taken in 1930; and (3) present rates of fault creep are far in excess of average prehistoric rates of land-surface offset.

End_of_Article - Last_Page 545------------