The Slichter Mode Detection and Estimation from Laser Interferometer–Strainmeter Observations of the Chilean Earthquake of February 27, 2010

Abstract—The Slichter mode (1S1) is the longest-period mode of the free oscillations of the Earth. The period of the Slichter mode directly depends on density jump between the outer liquid and the inner solid core which makes the detection of this oscillation very important for gaining a more detailed insight into the structure of the Earth’s interior. Reliable empirical data on the detection of Slichter mode are absent, which is associated with the rather low amplitude of this mode on the surface. In this work, for the first time, an attempt is made to detect the Slichter mode using the strain data from the largest 2010 Chilean earthquake recorded by the Baksan laser interferometer–strainmeter (Sternberg Astronomical Institute of the Moscow State University (SAI MSU)) with a measuring arm length of 75 m in the Elbrus region, North Caucasus. An asymptotically optimal data analysis algorithm that allows for the properties of seismic noise and peculiarities of the mode is developed. Simultaneously with mode detection, the algorithm provides evaluation of mode parameters (frequency and splitting magnitude). 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.