Characteristics of tidal gravity changes in Lhasa, Tibet, China

Abstract

Tidal gravity changes arise from the response of the solid Earth to the tidal forces of the Sun, Moon and planets close to the Earth, and are a comprehensive reflection of the structure and distribution of physical properties of the Earth's interior. As a result, observations of tidal gravity changes are the basis of studies on other global and/or regional dynamic processes. The characteristics of tidal gravity changes in the region of the Tibetan Plateau were investigated through continuous gravity measurements recorded with a superconducting gravimeter (SG) installed in Lhasa over a year. Through contrast measurements with a spring gravimeter LaCoste-Romberg ET20 at the same site, the gravity observations in Lhasa were scaled to the international tidal gravity reference in Wuhan. Meanwhile, the scale factor of the SG was determined accurately as -777.358 ± 0.136 nm s −2 V −1 , which is about 2.2% less than the value provided by the manufacturer. The results indicate that the precision of the tidal gravity observations made with the SG in Lhasa was very high. The standard deviation was 0.459 nm s −2 , and the uncertainties of for the four main tidal waves (i.e. O 1 , K 1 , M 2 and S 2 ) were better than 0.006%. In addition, the observations of the diurnal gravity tides had an obvious pattern of nearly diurnal resonance. As a result, it is affirmed that the Lhasa station can provide a local tidal gravity reference for gravity measurements on the Tibetan Plateau and its surrounding regions. The loading effects of oceanic tides on tidal gravity observations in Lhasa are so weak that the resulting perturbations in the gravimetric factors are less than 0.6%. However, the loading effects of the local atmosphere on either the tidal or nontidal gravity observations are significant, although no seasonal variations were found. After removal of the atmospheric effects, the standard deviation of the SG observations in Lhasa decreased obviously from 2.009 to 0.459 nm s −2 . Having removed the loading effects of oceanic tides and local atmosphere, it was found that the tidal gravity observations made with the SG in Lhasa significantly differed by about 1% from those expected theoretically, which may be related to active tectonic movement and the extremely thick crust in the region of the Tibetan Plateau. A more-certain conclusion requires longer accumulation of SG data and further associated theoretical studies.

Results of Gravity Observations Using a Superconducting Gravimeter at the Tibetan Plateau

“…Using the observed amplitude factors of tidal waves O 1 , K 1 and M 2 from the two gravimeters, the scale factor of SG-C057 was accurately determined as -777.358 ± 0.409 nm s -2 V -1 which is about 2.2% less than the value provided by the manufacturer (i.e., -795 nm s -2 V -1 ). The relative precision of calibration was as high as 0.05%, which completely satisfied the requirements for high-precision continuous gravity measurements (See Chen et al 2012 andXu et al 2012 for details).…”

“…The atmospheric gravity admittance was estimated as -3.631 ± 0.007 nm s -2 hPa -1 at the Lhasa station and was not significantly different from those either predicted by theoretical simulation or obtained with the SGs at the stations in other areas (Sun and Lou 1998;Sun et al 2001;Xu et al 2004aXu et al , 2008Xu et al , 2012. In order to intuitively show the local barometric pressure load effects upon the gravity measurement, Fig.…”

“…Figure 1 shows the location of the Lhasa station and the tectonic environment and surrounding region. The characteristics of the tidal gravity changes in Lhasa were investigated using continuous gravity measurements over more than one year recorded with the SG (Xu et al 2012).…”

Observational evidences for the speed of the gravity based on the Earth tide

“…The reason why we chose the three days shown in Table 1 is because during those days the phases of the moon are either 'new moon' or 'full moon'; in those days the Sun, the Earth and the Moon are almost collinear, and therefore the geometry between the Sun, Earth and Moon is much simple and reliable than other days during which the Moon is at other phases. In recent years, Xu et al [16] investigated the translational oscillation of the solid inner core of the Earth through gravity data recorded with superconducting gravimeters (SG); especially, they studied the tidal gravity change in Lhasa with SG measurements [17]. Obviously, the tidal gravity data from SG are more stable and accurate than those from the spring gravimeters; if using SG data in Lhasa, we may find the speed of gravity more accurately.…”

The observation of the Earth tide is irrelevant to the speed of gravity

“…They are far beyond the precision of the experiments even though the superconducting gravimeters are used as in [4][5][6]. In Newtonian theory, the space and time are flat and absolute.…”