Fault Systems of the Sigsbee Escarpment and Their Level of Late Quaternary Activity

The Sigsbee Escarpment in the northern Gulf of Mexico represents the seaward limit of the Sigsbee salt nappe and overlying supra-salt sediments. The escarpment is characterized by extensive faulting in the supra-salt sediments due to both gravitational potential of the escarpment and mobility of the underlying salt. An investigation of faulting above and below the escarpment was conducted to evaluate the potential for ground rupture hazard to planned subsea facilities.

The edge of the escarpment is affected by small-scale normal faults with displacement on the order of several meters to tens of meters. The base of the escarpment is affected by small-scale thrust faults with similar though generally smaller offsets. The faults are classified on the basis of forcing mechanism and their role in the formation of the escarpment. The resulting process-based structural model of the escarpment fault system and its relationship to salt movement is represented by multiple types of normal and thrust faults, both shallow and deep-seated.

The level of activity on the faults was evaluated for total displacement, magnitude of single events, and rate of movement of age-dated stratigraphic units. Uncertainty in these parameters was included as input to a probabilistic assessment of fault rupture hazard. The potential for both creep and episodic stick-slip mechanisms was also included in the analysis. Results indicate the slip rates are on the order of tenths of meters to several meters-per thousand-years with significant uncertainty in displacement mechanism. The overall hazard due to fault surface rupture is considered low.