Prediction of M_S 6.9 Menyuan and M_S 6.8 Luding earthquakes in 2022 based on gravity data

Several strong earthquakes (with magnitudes >= 6) occurred in western mainland of China in 2022. Two strike-slip earthquakes, an M-S 6.9 in Menyuan county, Qinghai Province, on January 8, and an M-S 6.8 in Luding county, Sichuan Province, on September 5, caused significant property damage, with the latter resulting in more than 100 casualties. For many years, the China Earthquake Administration has operated a gravity network near earthquake epicenters, with a number of relative and some absolute gravity stations serving as the main and auxiliary stations, respectively. In this study, the network was repeatedly monitored before the 2022 earthquakes; a large amount of relative gravity and a small amount of absolute gravity data were obtained, and changes in the regional gravity around the epicenter were determined. Changes in gravity fields are mainly reflected by relative gravity data, while absolute gravity data provide constraints. Based on the changes in gravity fields, we predicted the possibility of strong earthquakes of different magnitudes occurring in Menyuan county, Qinghai Province; Luding county, Sichuan Province; and their surrounding areas between 2021 and 2024. The distance between the epicenters of the two earthquakes recorded in 2022 and the center of the predicted earthquake risk zone in different years was < 60 km, with distances being < 10 km in 2021. Before the two earthquakes, positive and negative changes in surface gravity alternated around the epicenter and exhibited a four-quadrant distribution, one boundary axis of which coincided with seismogenic faults. The rupture zones of the earthquakes were located near the zero-value isolines of gravity changes. The maximum positive gravity change was > 90x10(-8) m s(-2)-Much greater than three times the observation error, while the minimum was approximately -40x10(-8) m s(-2). Both the amount of variation and the spatial extent of the positive changes were significantly greater than those of the negative ones. The spatial extent of gravity changes before both 2022 earthquakes was much larger than in the coseismic deformation zone, indicating that the spatial range of the seismogenic zone is much larger than that of the coseismic deformation zone. Compared with the distributions of the regions of expansion and compression reflected by the source mechanism solution and instantaneous coseismic deformation, the region characterized by positive gravity changes before an earthquake corresponded to coseismic compression, whereas negative changes corresponded to coseismic expansion. According to the elastic rebound theory of earthquakes, the regions with positive and negative gravity changes before the earthquakes are medium expansion and compression areas; these findings are consistent with the findings of the dilatancy-diffusion model. Based on data collected near Menyuan County, Qinghai Province, local gravity increased before the M-S 6.9 Menyuan earthquake, while the coseismic gravity changes over an interval of 6 months before and after the earthquake were negative. The temporal variation in local gravity for nearly four years before and after the M-S 6.9 Menyuan earthquake is also partially consistent with the results of the modified combined dilatancy model, which is based on the changes in gravity before and after the earthquake in the Beijing-Tianjin-Tangshan-Zhangjiakou region. Moreover, the slope of the slow wave time delay obtained by seismic shear wave splitting was the same as that of the gravity changes before and after the earthquake, and the moderate changes that caused the slow-wave time-delay also theoretically caused the gravity changes. These phenomena indicate that the gravity changes observed before the 2022 earthquakes should be helpful to develop earthquake precursor models.