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Maximum spectral energy for seismic magnitude estimation. Part I: Rayleigh-wave magnitude

Air Force Technical Applications Center, 1030 S. Highway A1A, Patrick AFB, Florida 32925-3002nazieh{at}rayleigh.aftac.gov

Abstract

A method for accurate estimation of surface-wave magnitudes is presented as an alternative to M_s, the classical time-domain magnitude. The salient features of the method are the use of velocity and frequency windows to isolate seismic phases and the use of maximum spectral energy to characterize ground motion for magnitude estimation. Rayleigh-wave ground motion estimated by the new method is generally consistent, but not identical, to that measured in the time domain. The estimated ground motion is used to determine Rayleigh-wave spectral magnitudes for 27 presumed nuclear explosions, from several test sites. These magnitudes are studied for their accuracy and for their transportability between nuclear explosion test sites. For accuracy, we compare spectral magnitudes M_R with their corresponding time-domain-measured magnitudes M_s. The comparison encompasses network magnitudes and their standard deviations and magnitude residuals. For transportability, we studied the variations in m_b (body-wave magnitude), M_s (time-domain magnitude), and M_R (spectral magnitude) for network-averaged m_b-equivalent presumed nuclear explosions at the Shagan River and the Qingger test sites. These studies showed that this spectral magnitude has a smaller error than classical time-domain M_s and that it is transportable between the two test sites.