Spectrum of seismic waves and its dependence on magnitude. I.

In order to study the dependence of a seismic spectrum on earthquake magnitude, spectral analysis for forty-three earthquakes is made by using an analog band-pass filtering method. A number of event pairs from nine seismic regions located in continents and island arcs are selected, and spectral ratios for two earthquakes from the same epicenter region but with different sizes are studied. The spectral ratio differs among the epicentral regions. Most of the results are generally in good agreement with the ω-square model proposed by Aki, but, some of them show ω0 or ω-1-dependece. This result is consistent with the observed regional variations of the mean Ms-mb, which was taken for the events that occurred for about four year. Though regional variations of the spectral ratio are not definitely detected, it is tentatively proposed that the main factor causing the difference may be due to the difference in the source time function among the event pairs.