Low-degree toroidal modes from the Sumatra-Andaman event observed by superconducting gravimeters

The complex frequencies of the low-degree toroidal modes are helpful in constraining the mantle structure. In the early years, the traditional method to detect the low-degree toroidal modes is seismic approach, which is primarily limited by the horizontal noise of seismometers. Recently, it has been proved that gravity method is competitive. Therefore, we re-estimated or newly estimated the complex frequencies of the toroidal modes below 1.5 mHz excited by the 2004 Sumatra-Andaman event using gravity method in this paper. The results show that (1) The quality factors of 0T5–0T9 and 1T2–1T3 are estimated from superconducting gravity data for the first time; (2) The estimated frequencies and quality factors for 0T2–0T9 and 1T2–1T3 are consistent with those provided by previous publications, which verifies the correctness and effectiveness of our estimations; (3) The maximum difference of frequencies between our results and those from CPREM is on the order of 0.01 mHz and the maximum difference of quality factors is at the level of 10. Keywords: Low-degree toroidal modes, Superconducting gravity data, Complex frequency, Quality factor