Preliminary interpretation of a seismic-refraction profile across the Large Aperture Seismic Array, Montana

A reversed seismic-refraction profile extending northeastward from Greycliff, Montana, across the Large Aperture Seismic Array (LASA) to Charleson, North Dakota, indicates that the crust of the earth consists of two layers with P-wave velocities of 6.1 km/sec and 6.7 km/sec, and that the upper-mantle velocity is 8.3 km/sec. The Mohorovicic discontinuity is 50-km deep at Charleson and remains at nearly the same depth southwestward for a distance of about 300 km, from where it slopes upward to the southwest at about 2° to a depth of 41 km near Greycliff, Montana.