Investigation of High-Frequency Noise for Superconducting Gravimeters of China Using Normal Modes of Lushan Earthquake

Given the very low instrumental noise at frequencies below 1 mHz, the superconducting Gravimeter (SG) is a highly sensitive and stable gravimeter for observations of the Earth's tides. As a new generation of SGs is widely used in observations and research on changes of gravity caused by activity in the interior Earth in recent years, the noise level of SGs at high frequency recording band is receiving more and more attention. Weak signals from the free oscillations of the Earth can help us directly investigate the capability of SGs to contribute to geodynamics research. The Lushan Mw6.6 earthquake, on 20 April 2013 in Sichuan, China, generated weak spheroidal modes in 2~8 mHz. We investigated the normal modes recorded by 5 SGs of mainland China, and computed the synthetic normal modes according to the theory of double couple centroid moment tensor based the spherical, non-rotating, elastic and isotropic (SNREI) Earth model. The results show that at the 2~5 mHz frequency band, the SG057 at the Lhasa station, the SG053 and the portable SG (iGrav) at the Wuhan station had a low noise level; the SG65 at Wuhan station and SG66 at Lijiang station had a high noise level. The test method and results can provide the reference when studying the high-frequency noise level of a gravimeter for clearing the application range.