A new method to measure seismic magnitude is introduced. The method is based on calculating the instantaneous amplitudes from equalized spectral amplitudes that are corrected for instrument response and filtered by a narrow band-pass filter. Rayleigh waves spectral magnitudes are obtained by taking the logarithm of the instantaneous amplitudes average in the 17- to 23-sec period range.

Rayleigh wave magnitudes for 22 Nevada Test Site nuclear explosions are calculated using the new method. When compared to available time-domain magnitudes, using the same network for each event, the standard deviation for the spectral magnitude averages are found to be lower than the standard deviation for the time-domain magnitude averages by more than 30 per cent. In general, the spectral magnitudes are lower than the time-domain magnitude by 0.3 to 0.5 magnitude units.

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