An Ms7.0 earthquake struck Jiuzhaigou (China) on 8 August 2017. The epicenter was in the eastern margin of the Tibetan Plateau, an area covered by a dense time-varying gravity observation network. Data from seven repeated high-precision hybrid gravity surveys (2014–2017) allowed the microGal-level time-varying gravity signal to be obtained at a resolution better than 75 km using the modified Bayesian gravity adjustment method. The “equivalent source” model inversion method in spherical coordinates was adopted to obtain the near-crust apparent density variations before the earthquake. A major gravity change occurred from the southwest to the northeast of the eastern Tibetan Plateau approximately 2 years before the earthquake, and a substantial gravity gradient zone was consistent with the tectonic trend that gradually appeared within the focal area of the Jiuzhaigou earthquake during 2015–2016. Factors that might cause such regional gravitational changes (e.g., vertical crustal deformation and variations in near-surface water distributions) were studied. The results suggest that gravity effects contributed by these known factors were insufficient to produce gravity changes as big as those observed, which might be related to the process of fluid material redistribution in the crust. Regional change of the gravity field has precursory significance for high-risk earthquake areas and it could be used as a candidate precursor for annual medium-term earthquake prediction.

Ground-based time-variable gravimetry with high accuracy is an important approach in monitoring geodynamic processes. The uncertainty of instruments including scale factor (SF) and drift rate are the primary factors affect the quality of observation data. Differing from the conventional gravity adjustment procedure, this study adopted the modified Bayesian gravity adjustment (MBGA) method, which accounts for the nonlinear drift rate, and where the SF is considered as one of the hyperparameters estimated using Akaike’s Bayesian information criterion. Based on the terrestrial time-variable gravity datasets (2018–2020) from the southeastern Tibetan Plateau, errors caused by nonlinear drift rate and SF were processed quantitatively through analysis of the gravity difference (GD) residuals and the mutual difference of the GD. Additionally, cross validation from absolute gravity (AG) values was also applied. Results suggest that: (1) the drift rate of relavive instruments show nonlinear characteristics, and owing to their different spring features, the drift rate of CG-5 is much larger than that of LCR-G gravimeters; (2) the average bias between the original and optimized SF of the CG-5 gravimeters is approximately 169 ppm, while that of the LCR-G is no more than 63 ppm; (3) comparison of the differences in gravity values (GV) suggests that the uncertainty caused by the nonlinear drift rate is smaller than that attributable to SF. Overall, the novel approach adopted in this study was found effective in removing errors, and shown to be adaptive and robust for large-scale hybrid surface gravity campaign which providing high accuracy gravity data for the geoscience research.

Extensometers housed in vaults are capable of resolving strain changes of less than a nanostrain; however, they are differently distorted by variations in barometric pressure at different frequencies, which may result in complicated strain noise. Therefore, how to quantify the barometric effects in different frequency bands is a perennial challenge in extensometric observation and research. Since the first strain observations at the Geodynamical Observatory Jingyuan (NW China) in 2007, the barometric effect has proven significant. Nevertheless, the frequency dependence and underlying mechanism of this effect remain unknown. With targeted research lacking at present, this study attempts to adopt the transfer function method to provide a systematic diagnosis. The results indicate that (1) the North–South (NS) component responds significantly to low-frequency pressure waves, with a slight phase shift leading the phenomenon; (2) the East–West (EW) component responds well to pressure waves in high-, medium-, and low-frequency bands, and the frequency dependence of the barometric coefficient spectrum and phase shift spectrum is strong; (3) the frequency-dependent response effect of the EW component may be related to factors such as horizontal pressure gradient force, topography, and fracture medium. These findings not only contribute to sub-frequency band correction of the barometric effect at the Geodynamical Observatory Jingyuan but also deepen our understanding of the ground–atmosphere coupling mechanism in the fault zone.