Abstract
The tidal and nontidal gravity change characteristics in the Tibetan Plateau region were investigated using the continuous gravity measurements recorded with a superconducting gravimeter (SG) installed in Lhasa from December 8, 2009 to September 30, 2011. The results indicated that the precision of the tidal gravity observations with the SG in Lhasa was very high. The standard deviation of the harmonic analysis for the gravity tides was 0.498 nm s^(-2), and the uncertainties of amplitude factors in the four main tidal waves (i.e., O1, K1, M2 and S2) were better than 0.002%. In addition, the diurnal gravity tide observations clearly revealed a pattern of nearly diurnal resonance. As a result, it is affirmed that the station should act as a local tidal gravity reference in the Tibetan Plateau and its adjacent regions. The load effects of oceanic tides are so weak that the resulting perturbation in the gravimetric factors is less than 0.6%. However, the load effects of the local atmosphere on either the tidal or the nontidal gravity observations are significant, although no seasonal variations have been found. After removing the atmospheric effects, the standard deviation of the harmonic analysis for the gravity tides decreased obviously from 4.160 to 0.498 nm s^(-2). Having removed the load effects of oceanic tides and local atmosphere, it is found that the tidal gravity observations are significantly different from those expected theoretically, which may be related to active tectonic movement and extremely thick crust in the Tibetan Plateau region. In addition, the Earth's free oscillations excited by 2011 Tohoku-Oki Mw 9.0 Earthquake were successfully detected.
Highlights
Ground-based continuous gravity measurements are a combined reflection of the transportation and exchanges of material and the deformation of the Earth, which are related to all kinds of environmental perturbations and geodynamic processes
The precision was better than 0.002% for the four main tidal waves and better than 0.02% for the other tidal waves with ampitudes exceeding 20 nm s-2
This implies that the tidal gravity observations made with the superconducting gravimeter (SG) at the Lhasa station were so accurate that they can act as a regional tidal gravity reference for the gravity measurements in the Tibetan Plateau and its surrounding regions
Summary
Ground-based continuous gravity measurements are a combined reflection of the transportation and exchanges of material and the deformation of the Earth, which are related to all kinds of environmental perturbations and geodynamic processes. Gravity data from a worldwide network of superconducting gravimeters (Crossley et al 1999; Crossley 2004; Crossley and Hinderer 2009) This project has produced numerous scientific achievements and benefited many disciplines. The Tibetan Plateau is located on the collision region of the Indian and Eurasian Plates. A permanent station of continuous gravity measurements was set up in Lhasa by the Institute of Geodesy and Geophysics, Chinese Academy of Sciences at the end of 2009 in order to investigate the related geodynamical hotspots such as Tibetan Plateau’s formation, evolution, uplifting rate and the related dynamic mechanism. The main motivation of this study is to investigate the tidal and non-tidal gravity changes, including accurate determinations of the gravimetric parameters, the load effects of oceanic tides and local atmosphere in the Tibetan Plateau, the Earth’s free oscillations, and the nearly diurnal resonance in the diurnal tidal gravity observations
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