Source Studies from Isolated Seismic Signals

Since 1960, new methods have been developed for extracting source information from surface-wave signals in the frequency range 2 mc/sec to 20 mc/sec. Recently, similar methods were successfully applied to body waves in the frequency range 10 mc/sec to 100 mc/sec. The author outlines the historical development of these ideas and demonstrates the present state of the art by a few examples which show clearly the success, as well as the limitations, of existing methods. Some new data are presented. Source mechanisms of the Queen Charlotte Island earthquake of 22 August 1949, the Sanriku earthquake of 2 March 1933, and the Iran shock of 1 September 1962 are revealed from analyses of mantle surface waves. As a counterexample, the source mechanism of a deep Banda Sea shock is derived from spectra of P and S waves recorded by stations of the World- Wide Standard Seismograph Network. Emphasis is placed on the derivation of the force system and the time function at the source by the phase-equalization method, and the source dimensions and rupture speed by the directivity method.