The Slichter Mode Detection and Estimation from the Observations of the February 27, 2010 Chilean Earthquake by a Laser Interferometer–Strainmeter
The Slichter mode (<sub>1</sub>S<sub>1</sub>) is the longest-period mode of the free oscillations of the Earth. The period of the Slichter mode directly depends on the density contrast between the outer liquid and the inner solid core, which makes its detection very important for gaining a more detailed knowledge of the structure of the Earth’s interior. The reliable empirical data on the detection of the mode are absent, which is due to the insignificant amplitude of the mode on the surface. In this work, for detecting the Slichter mode, we attempt to use for the first time the deformation data recorded during the largest 2010 Chilean earthquake by the Baksan laser interferometer–strainmeter of the Sternberg Astronomical Institute of the Moscow State University (SAI MSU). The instrument is located in the Elbrus region of the North Caucasus and has a measuring arm length of 75 m. An asymptotically optimal algorithm of data analysis is developed which takes into account the properties of the seismic noise and the peculiarities of the mode. Simultaneously with the detection of the mode, the algorithm allows for the evaluation of its parameters (frequency and splitting magnitude). The statistical reliability of the detection is estimated; the parameters of the Slichter mode and the corresponding density jump between the inner and outer core of the Earth are determined.