Earth’s Free Oscillations Excited by the 2013 Okhotsk Sea Earthquake

Based on the deformation data measured by the Baksan laser interferometer-strainmeter, Earth’s free oscillations (EFO) excited by the Okhotsk Sea deep-focus earthquake of May 24, 2013, the largest recorded deep-focus earthquake, are analyzed. The periods of 50 fundamental modes of EFO in the range 1.2–5.0 mHz are determined with an error of 3–12 μHz. The comparison of the EFO spectra for the May 24, 2013, Okhotsk Sea deep-focus earthquake the November 15, 2006, Simushir shallow-focus earthquake revealed a number of features of EFO excitation by the deep-focus earthquake. It is found that more overtones (both spheroidal and toroidal) are observed for the deep-focus earthquake. The amplitude values ​​of the observed EFO modes for the deep-focus earthquake are greater than for the shallow-focus earthquake, with a smaller observed deformation. The presence of interacting spheroidal and toroidal modes being close in frequency (coupling effect) is revealed. A method is proposed that can obtain an estimate of the EFO modes, which are not observed explicitly in the spectrum, from the beat period between close frequencies. Application of the method to the deformation data made it possible to find the frequencies of nine pairs of close EFO modes.