Seismic investigations of crustal structure in east-central Minnesota

Relative arrival times from 120 mine blasts recorded on the Central Minnesota Seismic Array (CMSA) out to distances of 350 km have been statistically analysed and interpreted to yield a crustal velocity-depth function for the Minnesota area. Velocities increase continuously from 5.9 km s^?1 at the surface to 7.4 km s^?1 at 40 km depth, with a steepening of the velocity gradient at about 20 km. A long-range refraction profile extending from the Mesabi Iron Range in Northern Minnesota to the seismic array, has also been recorded. A striking feature of the profile is the absence of direct P waves in the distance range 60–120 km. Our preferred explanation for this shadow zone is a fault or fracture zone near x = 60 km. Reflections within the shadow zone have been analysed to give a Moho depth of 42 km under northeastern Minnesota. Teleseismic residuals have been computed for 85 earthquakes recorded on the CMSA. No systematic dependence of the residuals upon azimuth or distance was found. The average residuals agree quantitatively with time delays predicted from earlier upper crustal refraction studies (Mooney et al., 1970).