Observations of the Singlets of Higher-Degree Modes Based on the OSE

Abstract

In this study, we select 18 SG (superconducting gravimeter) records from 15 GGP stations and 99 vertical, 69 horizontal components of IRIS broad-band seismograms during 2004 Sumatra Earthquake to detect the splitting of higher-degree Earth’s free oscillations modes (₀S₄, ₀S₇~₀S₁₀, ₂S₄, ₁S₅, ₂S₅, ₁S₆) and 12 inner-core sensitive modes (₂₅S₂, ₂₇S₂, ₆S₃, ₉S₃, ₁₃S₃, ₁₅S₃, ₁₁S₄, ₁₈S₄, ₈S₅, ₁₁S₅, ₂₃S₅, ₁₆S₆) by using OSE (optimal sequence estimation) method which only considers self-coupling. Results indicate that OSE can completely isolate singlets of high-degree modes in time-domain, effectively resolve the coupled multiplets independently, and reduce the possibility of mode mixing and end effect, showing that OSE could improve some signals’ signal-to-noise ratio. Comparing the results of SG records with seismic data sets suggests that the number of SG records is inadequate to detect all singlets of higher modes. Hence we mainly select plentiful seismograms of IRIS to observe the multiplets of higher modes. We estimate frequencies of the singlets using AR method and evaluate the measurement error using bootstrap method. Besides, we compared the observations with the predictions of PREM-tidal model. This study demonstrates that OSE is effective in isolating singlets of Earth’s free oscillations with higher modes. The experimental results may provide constraints to the construction of 3D Earth model.

Keywords

free oscillation / mode splitting detection / high-degree modes / OSE

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