FREE OSCILLATIONS OF THE EARTH OBSERVED BY A GALITZIN SEISMOGRAPH AT ABUYAMA, JAPAN

Abstract

The great Chilean earthquake of May 22, 1960 was observed with a Galitzin seismograph installed at Abuyama Seismological Observatory of Kyoto University, Japan. Only a vertical component of record was used in the present investigation. The record was read at every minute during a period of about 5.5 days beginning on May 19, 11 h 00 m, 1960 (UT). The whole reading was divided into five intervals in order to investigate the free oscillations of the earth excited by both the main shock and one of the large foreshocks of the Chilean earthquake. A low-cut filter was applied twice to all of the reading values, and then a Fourier analysis was made for every interval. It has come to be established that the free oscillations of the earth-spheroidal oscillations-could be detected by the present investigation and that their periods were in excellent agreement with periods obtained at other observation stations for the same earthquake and also with those predicted by theoretical investigations. An analysis was also made using data reread at 15 second intervals in order to investigate a fine spectral structure for lower modes of the spheroidal oscillations, and a splitting was found in spectral peaks corresponding to modes oS2, oS3 and oS4.