Surface-Wave Magnitude Calibration for European Region Earthquakes

This paper is concerned with the calibration of the surface-wave magnitude scale for the European region using the Prague formula. Two issues in particular have been addressed: ˙Does the Prague formula require a correction of the distance dependency term? ˙Is a source depth correction required? The issue of distance dependency has been investigated by examining the distance dependency of residuals from individual station Msmagnitudes and “representative” Ms magnitude values. We find that Ma magnitudes determined for the European region, using the Prague formula show slight dependence on recording station distance. The subsequent distance dependency of the mean Ma magnitude determination is therefore a function of the distance distribution of the recording stations. The question of depth correction has been investigated by examining the regression of Ms magnitudes versus seismic moment as a function of depth. We find that the Ms magnitudes determined by the Prague formula are only valid for crustal events, and require a depth correction for source depths greater than 30 km. As a by-product of these investigations, we find that for the European region there are stable nonlinear relations between surface-wave magnitude (Ms) and the log of seismic moment (logMo) and vice versa. These relationships will provide suitable functions for the correlation between one source size indicator and the other.