Objectives  High-precision coseismic three-dimensional (3D) deformation field is of great significance for investigating the earthquake deformation pattern and focal mechanism. For the 2021 Ms 6.4 Yangbi earthquake, the characteristics of coseismic deformation observation can be described as: (1) Only two directions of interferometric synthetic aperture radar (InSAR) line of sight observations from the ascending and descending tracks; (2) lack of azimuth observation; (3) some incoherent regions existed around the epicenter; (4) sparse and unevenly distributed GNSS observation. This observational fact makes it difficult to accurately extract its coseismic 3D deformation field. Therefore, a new method is investigated to solve this issue.

  Methods  The new method is designed and implemented by integrating earthquake dislocation model and extended simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements (ESISTEM) method. Firstly, the InSAR deformation field of the 2021 Ms 6.4 Yangbi earthquake is obtained by using the ascending and descending Sentinel-1A/B images. Secondly, the north-south deformation component obtained from the forward modeling of earthquake dislocation model is used as a constraint. Finally, with the new method, the complete coseismic 3D deformation field and strain field of the earthquake are successfully extracted.

  Results  (1) The 3D coseismic deformation field indicates that the southwest side of the Yangbi earthquake fault mainly moves westward and northward, with maxima of 4.8 cm and 9.5 cm, respectively. The northeast side mainly moves eastward and southward, with maxima of 7.4 cm and 4.6 cm, respectively. The maxima of vertical uplift and subsidence are 3.6 cm and 3.4 cm, respectively. (2) The earthquake fault is dominated by the right-lateral strike-slip movement with a small amount of normal components.(3) The strain field indicates that the earthquake fault area experiences significant expansion, shear, and rotation.

  Conclusions  The new method can make use of the north-south deformation component obtained from the forward modeling of earthquake dislocation model to carry out reasonable constraint, and take into account the spatial constraint between observation points. The reliability of the method is verified by the case study of the 2021 Ms 6.4 Yangbi earthquake. The method has reference value for expanding the method of extracting 3D deformation field.