Determination and interpretation of earthquake magnitude spectra

Abstract

Magnitude spectra are determined for three major earthquakes of the year 1985: the Xianjiang earthquake of 23 August, and the Mexico earthquake of 19 September with its largest aftershock of 21 September. Broad-band recordings obtained at the Central Seismological Observatory of the Federal Republic of Germany (GRF) are used for the analysis. Pass-band seismograms are obtained by way of filtering the broad-band seismogram. The magnitude spectrum of an earthquake is determined from the velocity amplitude for each Fourier component. The magnitude spectrum represents the velocity amplitude density spectrum at the earthquake source scaled in magnitude units. A comparison of the magnitude spectra shows significant differences between the focal parameters of the earthquakes, even if their conventional magnitudes (m_b , M_s ) are similar.