Free oscillations of the Earth observed on strain and pendulum seismographs

Abstract

Spectral analyses of seismograms of the great Chilean earthquake of May 22, 1960, from a newly installed strain seismograph at Ogdensburg, New Jersey, and from pendulum seismographs at Palisades, New York, have revealed spectral peaks corresponding to fundamental spheroidal modes 2 to 34, fundamental torsional modes 2 to 9, and the first overtone of the second spheroidal mode. Other peaks, some of which may be overtones, occur in the spectra but are not yet identified. Amplitudes of some observed spectral peaks vary radically between two time intervals on the same record. The peaks do not decrease in amplitude according to any simple law, and, rarely, an increase in amplitude with time is observed, indicating the apparent acquisition of energy in the mode between the time intervals. The periods of the graver modes of oscillation, both spheroidal and torsional, are in agreement with the theoretical values of Alterman, Jarosch, and Pekeris. The periods of the fundamental spheroidal oscillations between 250 and 500 seconds have been determined very accurately. These periods show excellent agreement with theoretical values calculated by Bolt and Dorman for a mantle with velocities according to the Gutenberg model and densities according to the Bullen model A. Also, phase velocities obtained from these periods are in agreement with Rayleigh wave phase velocities observed directly from the same earthquake by Brune, Nafe, and Alsop. Good agreement is also observed between torsional periods and theoretical values of Satô, Landisman, and Ewing, based on velocities of Jeffreys and densities of Bullen model A.

Tilt Observations in the Normal Mode Frequency Band at the Geodynamic Observatory Cueva de los Verdes, Lanzarote

“…The interval for background analysis was chosen just before the earthquake. The theoretical eigenfrequencies are indicated by vertical dashed lines and taken from ALSOP et al (1961).…”

Eigenvibrations of the Earth observed at Trieste

“…These authors also computed a spheroidal mode spectrum from the Kamchatka earthquake (1952 November 5 ) recorded by a vertical long-period seismograph at Palisades (New York). A frequency analysis (Alsop, Sutton & Ewing 1961) of data recorded by a strain seismograph at Ogdensburg (New Jersey) and by pendulum seismographs at Palisades (New York), revealed spectral peaks identified as fundamental spheroidal modes 2 to 3 4 and the first overtone of the second spheroidal mode. A long-period vertical seismograph operated at Chester (New Jersey) also recorded a series of 3 4 fundamental spheroidal modes, together with some spheroidal mode overtones (Bogert 1961).…”

Earth's free oscillations excited by the 2011 TohokuMw9.0 earthquake detected with a groundwater level array in mainland China

“…In particular, both spheroidal and torsional oscillations were revealed by a power spectral analysis of seismograms recorded by different seismographs. Alsop et al (1961) analysed periods of the graver modes of oscillations with a strain seismograph and a pendulum seismograph. Derr (1969) estimated an earth model through leastsquares inversion with weighted means of observed periods of free oscillations recorded by spring gravimeters.…”