Detection and Estimation of the ₁S₁ Slichter Mode from IGETS Superconducting Gravimeter Network Data after the 2011 Tohoku Earthquake

The ₁S₁ Slichter mode is the longest-period mode of the Earth’s free oscillations, caused by oscillations of the Earth’s inner solid core relative to the outer liquid core. In this paper, the search for and estimation of the Slichter mode from the IEGTS superconducting gravimeter network data after the 2011 Tohoku earthquake are performed. In the course of the work, the theoretical calculation of the Slichter mode splitting parameters for the PREM model was made. The mode was estimated using an original algorithm based on the maximum likelihood method. The algorithm uses optimal data integration obtained from all 21 records of 16 gravimeters, which can significantly increase the signal-to-noise ratio at the output of the detection system. Three most probable estimates of the degenerate frequency of the mode and its splitting parameters were obtained, what allows one to conclude that the Slichter mode was highly likely to be observed after the Tohoku earthquake. The differences in density between the inner and outer cores of the Earth corresponding to the periods of the mode were determined.