Eigenvibrations of the Earth after the Alaskan earthquake

Abstract

Spectrums of the free oscillations of the earth were computed from long-period pendulum seismograph recordings obtained at the Berkeley and Oroville seismographic stations after the Alaskan earthquake of March 28, 1964. Resolution was obtained for modal orders between l = 2 and l = 29. Longitudinal and transverse components of the ground motion at Oroville were synthesized by appropriate vector addition of the horizontal-component seismograms. Harmonic analysis of the separated components yielded spectrums of the torsional and spheroidal oscillations. A striking result was the appearance of the 0S12 mode in the spectrum of the longitudinal motion and its absence in the spectrum of the vertical motion. The Oroville station was therefore near a nodal line for the vertical component of the 0S12 mode. The variations of the spherical amplitudes at both the Berkeley and Oroville stations indicated an azimuthal order number m = 0. The average vertical ground amplitudes of several modes were estimated. Between 15 and 20 peaks on each spectrum were significant at the 95% confidence level. The measured eigenfrequencies are in close agreement with similar measurements from other earthquakes. Comparison with computed periods for four earth models showed that the Gutenberg-Bullen A model gives the most satisfactory agreement with the observations.