Heping Sun

Tidal gravity observations obtained with a superconducting gravimeter at Wuhan/China and its application to geodynamics

Abstract The long period tidal gravity observations recorded with a superconducting gravimeter at station Wuhan/China from 1 January 1986 to 31 December 1994 (old series) and from 20 December 1997 to 30 April 2000 (new series) are studied comprehensively. The new tidal amplitude factors and phase differences are determined precisely using modern international standard data processing techniques. The tidal gravity signals due to the geophysical sources as of global ocean tides and local atmospheric pressure are comparatively studied. The dynamic influences of the Earths core, i.e. the resonance parameters (eigenperiod, quality factor and resonance strength) of the free core nutation, are determined and the corresponding results are compared with those obtained in the previous studies.

Experimental earth tidal models in considering nearly diurnal free wobble of the Earth＇s liquid core

Based on the 28 series of the high precision and high minute sampling tidal gravity observations at 20 stations in Global Geodynamics Project (GGP) network, the resonant parameters of the Earths nearly diurnal free wobble (including the eigenperiods, resonant strengths and quality factots) are precisely determined. The discrepancy of the eigenperiod between observed and theoretical values is studied, the important conclusion that the real dynamic ellipticity of the liquid core is about 5% larger than the one under the static equilibrium assumption is approved by using our gravity technique. The experimental Earths tidal gravity models with considering the nearly diurnal free wobble of the Earths liquid core are constructed in this study. The numerical results show that the difference among three experimental models is less than 0.1%, and the largest discrepancy compared to those widely used nowdays given by Dehant (1999) and Mathews (2001) is only about 0.4%. It can provide with the most recent real experimental tidal gravity models for the global study of the Earths tides, geodesy and space techniques and so on.

International tidal gravity reference values at Wuhan station

The international tidal gravity reference values at Wuhan station are determined accurately based on the comprehensive analysis of the tidal gravity observations obtained from 8 instruments. By comparing these with those in the tidal models given by Dehant (1997) while considering simultaneously (i) the global satellite altimeters tidal data, and (ii) the Schwiderski global tidal data and the local ones along the coast of China, it is found that the average discrepancy of the amplitude factors and of the phase differences for four main waves are given as 5.2% and 3.6% and as 0.16° and 0.08° respectively. They are improved evidently compared to those determined in early stage, indicating the important procedures in improving the Wuhan international tidal gravity reference values when including the long-series observations obtained with a superconducting gravimeter, and when considering the influence of the ocean loading and of the nearly daily free wobble of the Earth’s core.

Wavelet approach to the determination of gravity tide parameters

A new approach is proposed for the determination of gravity tide parameters. Three pairs of compactly supported wavelet filters are introduced in the approach. They can efficiently extract the objective tides from the gravity observation series. The new approach guarantees a direct and precise analysis on the tidal gravity records of any sampling length. The new approach is applied to the harmonic analysis on Wuhan superconducting gravimeter records. The results clearly show the resonant effects of the Earth Nearly Diurnal Free Wobble (NDFW).

Constraining the focal mechanism of the Lushan earthquake with observations of the Earth’s free oscillations

The amplitudes of the Earth’s free oscillations have a close relationship to earthquake focal mechanisms. Focal mechanisms of large earthquakes can be well analyzed and constrained with observations of long period free oscillations. Although the 2013 Lushan earthquake was only moderately sized, observable spherical normal modes were excited and clearly observed by a superconductive gravimeter and a broadband seismometer. We compare observed free oscillations with synthetic normal modes corresponding to four different focal mechanisms for the Lushan earthquake. The results show that source parameters can be analyzed and constrained by spherical normal modes in a 2.3–5 mHz frequency band. The scalar seismic moment M0 has a major influence on the amplitudes of free oscillations; additionally, the strike, dip, rake and depth of the hypocenter have minor influences. We found that the synthetic modes corresponding to the focal mechanism determined by the Global Centroid Moment Tensor show agreement to the observed modes, suggesting that earthquake magnitudes predicted in this way can readily reflect the total energy released by the earthquake. The scalar seismic moment obtained by far-field body wave inversion is significantly underestimated. Focal mechanism solutions can be improved by joint inversion of far- and near-field data.

Detection of inner core translational oscillations using superconducting gravimeters

An attempt has been made to search for the translational oscillations of the Earth’s solid inner core in the gravity measurements recorded with the superconducting gravimeters (SG) from the worldwide network of the Global Geodynamics Project (GGP). All the SG data were prepared and analyzed by the same method to remove accurately the signatures related to gravity tides, local barometric pressure, the Earth’s rotation, the long-term trend and so on. We obtained the estimations of the power spectral densities of each residual series and the estimations of the product spectral densities in the subtidal band (0.162–0.285 cph) were obtained by using a multi-station stacking technique after further eliminating atmospheric effects. The inner core translation triplet was detected in the subtidal band. We find 6 groups of signal with high signal-to-noise ratio that are consistent with the characteristics of the triplet, and 4 groups of the results that are close to the previous studies and the differences are less than 0.92%. It implies that the groups of signatures all have the possibility to be related to the inner core translational oscillations.

Application of superconductive gravity technique on the constraints of core-mantle coupling parameters

The parameters, i.e. the Period and the Quality factor, of the Earth’s free core nutation (FCN) are closely related to the dissipative coupling between the core and the mantle. Based on the FCN parameters obtained from the actual observations and theoretical simulation, significantly constrained in this study were several key parameters near the core-mantle boundary (CMB), related to the core and mantle coupling, including viscosity at the top of liquid core, conductivity at the bottom of the mantle, and dynamic ellipticity of the CMB. In order to choose high quality observations from global stations of the superconducting gravimeters (SG) on the Global Geodynamics Project (GGP) network, we adopted two criteria, the standard deviations of harmonic analysis on tidal observations and the quality of the FCN parameters calculated with the observations from single station. After the mean ocean tidal effects of the recent ocean tidal models were removed, the FCN parameters were retrieved by stacking the tidal gravity observations from the GGP network. The results were in a good agreement with those in the recent research by using the SG and/or the VLBI observations. Combined with an FCN theoretical model deduced by angular momentum method, the viscous and electromagnetic coupling parameters near the CMB were evaluated. Numerical results indicated that the viscosity at the top of the liquid core was in the range from 6.6×102 to 2.6×103 Pa·s, which was in good agreement with those obtained from the Earth’s nutation, the FCN and variations in the length of day (LOD). The conductivity at the bottom of the mantle should be as large as 2.6×106–1.0×107 S m−1 to match the FCN quality factors from the actual observations. The dissipative coupling had a little influence of 1–2 sidereal days for the FCN period.