STRESSING OF THE NEW MADRID SEISMIC ZONE BY A LOWER CRUST DETACHMENT FAULT

A new mechanical model for the cause of the New Madrid seismic zone in the central United States is analyzed. The model contains a subhorizo ntal detachment fault which is assumed to be near the domed top surfac e of locally thickened anomalous lower crust (''rift pillow''). Region al horizontal compression induces slip on the fault, and the slip crea tes a stress concentration in the upper crust above the rift pillow do me. In the coseismic stage of the model earthquake cycle, where the th ree largest magnitude 7-8 earthquakes in 1811-1812 are represented by a single model mainshock on a vertical northeast trending fault, the m odel mainshock has a moment equivalent to a magnitude 8 event. During the interseismic stage, corresponding to the present time, slip on the detachment fault exerts a right-lateral shear stress on the locked ve rtical fault whose failure produces the model mainshock. The sense of shear is generally consistent with the overall sense of slip of 1811-1 812 and later earthquakes. Predicted rates of horizontal strain at the ground surface are about 10(-7) year(-1) and are comparable to some o bserved rates. The model implies that rift pillow geometry is a signif icant influence on the maximum possible earthquake magnitude.