Quantifying of spatio-temporal variations in the regional gravity field and the effectiveness of earthquake prediction: A case study of M_S ≥ 5.0 earthquakes in the Sichuan-Yunnan region during 2021–2024

Since the 1975 M_S7.3 Haicheng earthquake, spatio-temporal variations in the gravity field have attracted much attention as potential earthquake precursors. Recent technical advances in terrestrial gravity observation, along with the construction of a high-precision mobile gravity network covering Chinese mainland, have positioned temporal gravity variations (GVs) as an important tool for clarifying the signal characteristics and dynamic mechanisms of crustal sources. Reportedly, crustal mass transfer, which is affected by stress state and structural environment, alters the characteristics of the regional gravity field, thus serving as an indicator for locations of moderate to strong earthquakes and a seismology-independent predictor for regions at risk for strong earthquakes. Therefore, quantitatively tracking time-varying gravity is of paramount importance to enhance the effectiveness of earthquake prediction. In this study, we divided the areas effectively covered by the terrestrial mobile gravity network in the Sichuan-Yunnan region into small grids based on the latest observational data (since 2018) from the network. Next, we calculated the 1- and 3-year GVs and gravity gradient indicators (amplitude of analytic signal, AAS; total horizontal derivative, THD; and amplitude of vertical gradient, AVG) to quantitatively characterize variations in regional time-varying gravity field. Next, we assessed the effectiveness of gravity field variations in predicting earthquakes in the Sichuan-Yunnan region using Molchan diagrams constructed for gravity signals of 13 earthquakes (M ≥ 5.0; occurred between 2021 and 2024) within the terrestrial mobile gravity network. The results reveal a certain correspondence between gravity field variations and the locations of moderate and strong earthquakes in the Sichuan-Yunnan region. Furthermore, the 3-year AAS and AVG outperform the 3-year THD in predicting subsequent seismic events. Notably, the AAS and AVG showed large probability gains prior to the M_S6.8 Luding earthquake, indicating their potential for earthquake prediction.