Amplitude spectra of PcP and P phases

Abstract

Amplitude spectra were obtained from short-period PcP and P phases from seven explosions and six earthquakes. Long-period PcP and P amplitude spectra were obtained from two earthquakes. PcP and P amplitude spectra for both explosions and earthquakes are similar for any one event; therefore, station and core-mantle boundary effects are small and the general shape of the spectra is related to the source. All of the explosions studied have characteristic spectra with a pronounced minimum in the spectrum near one second. The period of this minimum increases with magnitude of the event. Short-period amplitude spectra from some intermediate- and deep-focus earthquakes resemble those from explosions. Spectra from the other earthquakes studied differ markedly from those of explosion; they have either no minimum in the spectrum near one second or very little energy for periods less than one second. The characteristics of the spectra may be of help in the classification of sources. On a plot of magnitude m_b versus period of the minimum T_d in the spectrum of explosions, the data form a straight line. Earthquakes with an amplitude spectrum similar to that of an explosion are randomly distributed on the plot of m_b versus T_d. Systematic effects of focal depth were not observed. Layering at the coremantle boundary was not detected.