Studies with a small seismic array in eastcentral Minnesota

Relative arrival times from 120 mine blasts at distances of 80 to 350 km from the Central Minnesota Seismic Array were statistically analyzed to yield a crustal velocity-depth function for the area. A long range refraction profile, extending from the Mesabi Iron Range in northern Minnesota to the seismic array, was also carried out. A striking feature of the profile is the absence of direct P waves in the distance range 60 to 120 km, suggesting the presence of a fault or fracture zone near 60 km. Dispersion relations for the fundamental mode Rayleigh wave, observed on Mesabi mine blast record, were interpreted to reveal a 1.2 km thick surface layer of shear velocity 2.7 km/s overlying a half space of shear velocity 3.3 km/s. The geologic interpretation of the model is that of the Animikie Group metasediments overlying the Lower Precambrian gneisses and granities.