Detection and Estimation of the Slichter Mode \(\mathbf{{}_{1}S_{1}}\) from Strain Data from the Baksan Laser Interferometer after the 2011 Tohoku Earthquake

The \({}_{1}\textrm{S}_{1}\) mode of the Earth’s free oscillations (Slichter mode) arises from the oscillations of the Earth’s inner solid core relative to the outer liquid core. This study conducts the search for and estimation of the Slichter mode following the 2011 Tohoku earthquake based on data from the Baksan laser interferometer–strainmeter. An algorithm based on the maximum likelihood method (MLM) was used to detect and estimate the mode parameters. The most probable estimates of the degenerate mode frequency and its splitting parameters have been obtained, which may be associated with the excitation of the Slichter mode in the analyzed data. An estimate of the density jump between the Earth’s inner and outer core has been derived for the found mode period. For the first time, a comparison has been made between the potential capabilities of the Baksan strainmeter and the superconducting gravimeters of the IGETS network for the task of observing the Slichter mode after the same earthquake. The similarity of the obtained mode estimate from strain observations with the previously obtained estimate from gravimetric observations, as well as with the theoretical value from the PREM model, is noted.