A case study of detecting the triplet of 3S1 using superconducting gravimeter records with an alternative data preprocessing technique

Due to their very low noise levels in the low frequency band (<1 mHz), superconducting gravimeters (SGs) are particularly suitable to observe long-period free oscillations of the Earth. This case study is dedicated to the detection of the triplet of the seismic normal mode 3S1 that was excited by the December 26, 2004, Sumatra-Andaman earthquake (Mw = 9.3). Some SG records and the Hilbert-Huang transformation are used as an alternative data preprocessing technique, instead of the traditional detiding method. After removal of atmospheric pressure effects from the original SG records, we applied the Hilbert-Huang transformation to the SG residues, to select the signals that included the frequency band of interest, and to construct a new data series. Then, by applying the multi-station experimental technique to five 273-h-long common new data series recorded at different SG stations, we clearly observed all of the three singlets of the mode 3S1, with the central singlet more evident compared to previous studies. Observations of the low-frequency modes 3S1 (n = 0, 1, 2, ...; l = = 1,2, ...) provide constraints on the inner and outer core structure. This case study provides an alternative data-preprocessing approach to observe the splitting frequencies of the low-frequency mode type 3S1 (n = 0, 1, 2, ...).