Magnitude calibration for the Central Minnesota Seismic Array presents special problems because the infrequuent local and near-regional earthquake are usually too small to be recorded at teleseismic distances. Two procedures have been applied. The first uses teleseisms recorded by the array for which independent magnitudes are available. The second uses large near-regional mine blasts to determine the attenuation rate for short-period P waves in the upper crust, together with Evernden's (1970) magnitude-charge size relationship.
The teleseismic correlations establish that the individual stations of the array yield consistent magnitudes, that the P wave amplitudes behave predictably with respect to epicentral distance and azimuth, but that a magnitude-dependent bias must be removed.
Measured ground amplitudes for mine blasts are found to be proportional to total charge size to the power 1.0. Using this value, we find amplitude attenuation proportional to D−B for distance D in kilometers, with best fit given by B = 2.57 for P waves. The final local magnitude scale for D up to 250 km takes the form