Time-Variable Gravity Observations from the Chinese Mainland Seismic Gravity Network Reveal Deep Mass Migration during Large-Earthquake Preparation

With the advancement of high-precision absolute gravimeters, absolute/relative hybrid observation systems now enable the detection of terrestrial time-varying gravity signals, offering insights into density changes and mass redistribution within seismogenic zones. However, extracting weak gravity signals related to earthquakes remains challenging due to complex surface processes and unclear mechanisms. This study investigates gravity changes before three major earthquakes, such as the 2013 Lushan Ms7.0, 2021 Yangbi Ms6.4, and 2022 Menyuan Ms6.9 events on the eastern Tibetan Plateau. To address issues such as nonlinear drift and scale-factor variability in relative gravimeters, a Bayesian gravity adjustment method is introduced to enable quantification of observational uncertainties. Then, after correcting for hydrological and vertical deformation effects, residual gravity changes possible related to deep tectonic mass transport are derived. For quantitative description, a mass source model inversion method is used to quantify potential links between gravity changes and crustal mass transfer in seismogenic regions. Finally, combined with historical seismicity, velocity, and electrical structure models, we constructed several mass source models for understanding the mechanism of deep mass migration before strong earthquakes.