Abstract:Using 54 GNSS reference station observations in and around Yunnan from May 20, 2019 to May 20, 2021, we obtain the velocity field and strain field in Yunnan based on the least squares configuration method.We combine the time series of GNSS baseline length changes in the near-field and far-field regions of the 2021-05-21 Yangbi MS6.4 earthquake to analyze the crustal deformation characteristics before the earthquake. The results show enhanced motion in the SSW direction at the western boundary of Yunnan before the earthquake, and a dispersive motion along the Nujiang fault. The near-field velocity values near the epicenter did not change much, and the anomalous changes in the regional velocity field are not obvious. The value of the surface expansion rate of the study area as a whole before the earthquake dropped to about 0.37 times its original value, while the maximum shear strain rate and principal strain rate also dropped sharply, indicating that the crustal strain accumulation in the area before the earthquake weakened significantly, and the epicenteris located on the zero value line of alternating tension and compression.The GNSS baseline length time series results show that the GNSS baseline anomalies in the near-field region before the earthquake changed insignificantly, and the GNSS baseline in the far-field region has trend anomalous changes that deviate from the long-term background before the earthquake. The baseline KMIN-YNYM accelerates backward in tension against the background of trend compression, deviating from the trend motion by 8.4 mm. The baseline YNLP-KMIN accelerates in compression against the background of trend compression, deviating from the trend motion by 6.4 mm; the baseline YNLP-YNDZ accelerates in compression against the background of relatively smooth trend deformation, deviating from the trend motion by 8.1 mm. The baseline YNSM-YNLP accelerates in tension against the background of trend tension, deviating from the trend motion by 7.8 mm. Weinferthat the stress equilibrium state in the region before the earthquake was broken and induced the earthquake.