Station Magnitude Bias - Its Determination, Causes, and Effects

An analysis of some 400,000 stations m sub b values as reported in the ISC bulletin reveals substantial global variations in station magnitude bias, defined as the mean difference between station m sub b and the average m sub b of a large network of stations. Although there are clear indications that the biases are functions of both source region and time, they appear to be well correlated with tectonic structure and lateral variations in attenuation characteristics in the upper mantle under the station. Application of these biases as station magnitude corrections reduces the scatter in m sub b observations for a single event. Changes in station distribution with time are shown to introduce perceptible temporal changes in the shape of magnitude-frequency curve; these can be greatly reduced by application of the station corrections. These corrections, through their effect on the M subs mb sub b diagram and the magnitude-yield relation, are applicable to the problems of seismic discrimination.