Seismicity-Based Clues of Crustal Fluids in the 2021 M6.4 Yangbi Earthquake Sequence, Yunnan, China

Summary On 21 May 2021, a moderate earthquake with a magnitude of M6.4 occurred in western Yunnan Province, China. Several felt earthquakes had occurred in the region in the three days preceding this event. We aimed to investigate whether there are indications of crustal fluid migration in this region and, if so, the extent to which regional seismicity might be influenced by crustal fluids. Using the epidemic-type aftershock sequence (ETAS) model, we analyzed the earthquake triggering process and the nonstationary background rate. In addition, seismicity rates were used to estimate Coulomb stress changes. Specifically, the results suggest that the background rate increased from 0.2 to 15 events per day during the Yangbi earthquake sequence, and the spatial evolution of the background rate was broadly consistent with a fluid diffusion pattern. The evaluated cumulative stress change indicates that the stress rate increased by approximately 50 per cent following the M6.4 earthquake. Spatial stress changes suggest migration and expansion of regions with increased stress. These results imply the presence of crustal fluid flow in the study area, with a tendency to diffuse southward.