In a 1977 study of teleseismic events recorded by SLU instruments in the New Madrid zone we observed a systematic regional variation of travel times that indicated that subsurface velocity structure varied laterally. Later work by Haydar Al-Shukri and I showed that these variations in the upper mantle largely coincide with the location of mapped intrusives and upper crustal variations show a striking correlation with earthquake activity. Upper crustal velocities are lowest where earthquake activity is greatest.

Extending ideas in my dissertation of 17 years earlier, we interpreted the reduced velocities as being due to fluids in faults, fractures, and permeable rock in the active portion of the New Madrid seismic zone. These fluids also serve to reduce effective stress in those regions so that earthquakes can more easily occur.

Mitchell, B.J., C.C. Cheng, and W. Stauder, A three-dimensional velocity model of the lithosphere beneath the New Madrid seismic zone, Bull. Seism. Soc. Am., 67, 1061-1074, 1977.

Al-Shukri, H., and B.J. Mitchell, Three-dimensional velocity variations and their relation to the structure and tectonic evolution of the New Madrid seismic zone, J. Geophys. Res., 92, 6377-6390, 1987.

Al-Shukri, H., and B.J. Mitchell, Reduced seismic velocities in the source zone of New Madrid earthquakes, Bull. Seism. Soc. Am., 78, 1491-1509, 1988.

Al-Shukri, H., B.J. Mitchell, and H. Ghalib, Attenuation of seismic waves in the New Madrid seismic zone, Seism. Res. Lttrs., 59, 133-140, 1989.

Mitchell, B.J., O.W. Nuttli, R.B. Herrmann, and W. Stauder, Seismotectonics of the central United States, in Neotectonics of North America, edited by D.B. Slemmons, E.R. Engdahl, D. Blackwell, and D. Schwartz, Geol. Soc. Am., Boulder, CO, 245-260, 1991.