Detection of the Slichter mode triplet using superconducting gravimetric observations

Based on 13200-hour records by nine superconducting gravimeters (SGs) under a worldwide network of the Global Geodynamics Project after the 2004 Sumatra earthquake, a series methods including the ensemble empirical mode decomposition (EEMD), the optimal sequence estimation (OSE), the product spectrum analysis (PSA) and the autoregressive (AR) are applied jointly to detect the Earth's inner core translational mode triplet (Slichter mode triplet). We first obtain the intrinsic mode functions (IMFs) of the target frequency band extracted from the residual sequence after the routine pretreatment as the final gravity residual, and each of the nine residual gravity series is divided into three segments of the same length without overlap, and the product spectra of Slichter triplet are obtained by using OSE and PSA. Then, the AR method is used to estimate the central frequencies of the week resonance signals and the corresponding errors. The results show that three clear spectral peaks are detected using OSE and PSA, which correspond respectively to the singlets m=-1, m=0 and m=+1. The periods corresponding to the three spectral peaks are close to the prediction of the PREM model given by Crossley (1992), Rochester and Peng (1993), Peng (1997) and Rogister (2003), as well as the "possible observation results" suggested by Ding and Shen (2013). Referring to the existing theory and observed results of the Slichter triplet, we conclude that the three observed signals in this study might correspond to the Slichter triplet.