The translational oscillation of the solid inner core is one of the Earth’s fundamental normal modes, which is also called Slichter mode. The normal mode should be split to form a triplet due to the Earth’s rotation and ellipticity. In this study, according to the splitting pattern of Slichter mode, an attempt has been made to detect the possibility of Slichter triplet’s existence by using the product spectra of the long-term continuous gravity measurements from the superconducting gravimeters (SG) at 6 glob- ally-distributed permanent stations in the Global Geodynamics Projects network. The results indicate that the background noise level of the global SG observations is 0.0158 nm s–2 and the magnitude threshold of any global harmonic signals, which may be detected by the global SG, is 0.0152 nm s–2 in the subtidal frequency band from 0.162 to 0.285 cph in which Slichter triplet may occur. it implies that the signatures, related to the triplet, may be identified in the global SG observations, if they exist. It is found that there is a group of global harmonic signatures with the periods of 5.310, 4.995 and 4.344 h emerging significantly from the background noise in the global SG observations. They are in good agreement with the splitting pattern of Slichter mode. It implies that this group of signatures may be related to the inner core translational oscillations. The associated density contrast across the inner core boundary may be deduced as between the values provided in the Earth models of the PREM and 1066 A.
The mode serials of the Earth’s free oscillation provide some important information on the Earth’s deep structure and superconducting gravimeters （SG） can investigate the phenomena of the Earth’s free oscillations with high accuracy. The great Sumatra-Andaman earthquake fully excited the Earth’s free oscillations and these signals were perfectly recorded by five superconducting gravimeters in the globe. After the pre-treatment and spectral analysis on the SG observation data, we obtained the experimented mode serials of the Earth’s free oscillations consisting of 147 modes with GGP station data. These observed modes were themselves some new important data for the study of the Earth’s deep structure. On the basis of the discussions on some checked inner-core-sensitive modes, we distinguished three layers from the inner core, and the boundary of the upper layer was compatible with the formerly known transition zone in the inner core based on seismic body waves and supported that there were the hemispherical variation and very lower shear velocity zone in the lower inner core.
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.
The Earth＇s free core nutation （FCN） is a retrograde eigenrnode which is attributed to the interaction between the solid mantle and the liquid core of the rotational elliptical Earth. This mode appears as an eigenmode of nearly diurnal free wobble （NDFW） in a terrestrial reference frame with a period of about one day （XU et al, 2001）. Therefore, the NDFW will lead to an obvious resonance enhancement in the diurnal tidal gravity observations, especially those of the tidal waves with frequencies closed to its eigenfrequency such as P1, K1, ψ1 and Ф1. The FCN resonance parameters can be retrieved accurately by high-precision tidal gravity observations, especially those recorded with the superconducting gravimeters （SG）. The Global Geodynamics Project （GGP） organized by IUGG took it as an important content for determining the FCN resonance parameters by using gravity data. However, the results are affected by many factors such as station location, background noise, the selection of the tide-generating potential tables, ocean tide models, data processing techniques and so on. In our study, the FCN parameters will be retrieved by using the SG observations at Wuhan, and the effects of the choices of various tide-generating potential tables, oceanic models and weight functions on the estimation of the FCN parameters will be discussed in detail,
Although it is well known that coseismic gravity changes take place during an earthquake, previous research has not yielded convincing evidence demonstrating that significant gravity changes occur before large earthquakes. Furthermore, even if we suspect that gravity changes occur before large earthquakes, we have yet to demonstrate how to consistently observe these changes for useful earthquake forecast that would bring benefits to society. We analyzed ground gravity survey data obtained in 1998, 2000, 2002, and 2005 at stations of the Crustal Movement Observation Network of China (CMONOC) and examined gravity changes be-fore the occurrence of nine large (Ms≥6.8) earthquakes that ruptured within or near China's Mainland and Taiwan from November 2001 to August 2008. Results from this analysis show that significant gravity changes occurred across a large region before each of these nine large earthquakes, and these changes were detected by repeated ground gravity surveys through CMONOC. Although these gravity changes were significant, more research is needed to investigate whether these gravity changes could be viewed as precursors of large earthquakes. Limitations and uncertainties in the data include sparseness of the gravity monitoring network, long time intervals between consecutive gravity surveys, inevitable measurement errors, hydrological effects on gravity, and effects of vertical crustal movements on gravity. Based on these observations, we make several recommendations about possible future direc-tions in earthquake-related research using gravity monitoring data.
ZHAN F. BenjaminZHU YiqingNING JinshengZHOU JiangcunLIANG WeifengXU Yunma
We consider the characteristics of long-term changes in non-tidal gravity and their implication to the local perturbations in barometric pressure and water storage and to the local vertical crustal movement using the long-term continuous gravity observations from a superconducting gravimeter (SG) at Wu-han station,together with the co-site measurements from a Global Positioning System (GPS) receiver and an absolute gravimeter FG5. The observation results indicate that there are obvious seasonal variations in the long-term gravity changes measured with the SG. About 70 percent of the whole sea-sonal changes come from the contribution of the local disturbances in air pressure and water storage,while over 95 percent of the annual changes are attributed to the loading effects of these environmental perturbations. Due to the absence of direct measurements of the local water storage,especially those of the underground water,the global assimilating models of land water LaD (Land Dynamics) and GLDAS (Global Land Data Assimilation System) cannot virtually describe the real hydrologic distur-bances around the station. The resulting gravity changes,which are simulated theoretically by means of convolution integration of the loading Green’s functions and water models LaD and GLDAS,show significantly time delay of about 55 days from those measured with the SG. Compared with the meas-urements of the absolute gravity with the FG5,the long-term drift rate of the SG is determined as about 17.13 nms-2/a. From the co-site GPS measurements,it is found that the local crust is slowly subsiding at a rate of 3.71±0.16 mm/a,and the related gravity variation is estimated as 13.88±0.22 nms-2/a. In other words,the ratio of the changes in gravity and altitude related to the local vertical crustal movement is about -37.41 nms-2/cm. It implies that a considerable mass adjustment may be associated with the local vertical crustal movement,and its dynamic mechanism should be investigated further.
The synthetic tidal parameters with high spatial resolution for gravity over China and its neighbor area are constructed with Earth＇s tidal model and ocean tide loading calculated using TPXO7 global ocean tide model as well as tidal data over China seas. The comparison between synthetic parameters and ones observed by spring gravimeters at some seismic network stations and Hong Kong station and one observed by super-conducting gravimeter at Wuhan station shows that the average differences in amplitude factors and phases are smaller than 0.005 and 0.5° respectively; and that the discrepancies between observational and synthetic parameters are dependent on gravimetric technique in that the synthetic parameters are in well agreement with the superconducting gravimetric observations. This also indicates that the synthetic result is a good estimation for tidal gravity, and the numerical results in the present paper not only can provide ground and space gravimetry such as absolute gravimetry with correction model of tidal gravity, but also provide effective tidal parameters over areas where no observation is carried out.
