Objectives  At 15:14 on October 17, 2016 (Beijing time), an earthquake with a magnitude of Mw 5.9 occurred in Zadoi County, Qinghai Province, which is located in the Qiangtang block in the middle of the Tibetan Plateau. The occurrence of the event provides an opportunity to deepen the understanding of the movement characteristic of regional fault zone. The fault slip model of the Zadoi earthquake is further investigated with geodetic observation to understand the fault rupture property of the event better. The rupture characteristic of typical faults around the epicenter is quantitatively studied using numerical method, which can make up for the inadequacies of qualitative interpretation of seismic fault rupture using feature of earthquake sequence and regional geological structure in the previous studies.

  Methods  To study this earthquake, coseismic deformation field was obtained using interferometric synthetic aperture radar (InSAR) technique and Sentinel-1 satellite ascending and descending synthetic aperture radar (SAR) data. On the basis of comparative analysis of existing studies related to rupture model of seismic fault, the fault slip model of this event was then inverted with the elastic half-space dislocation theory and non-negative least squares method. The average rakes of typical faults around the epicenter under regional strain field were calculated with boundary element method, and the earthquake fault slip was finally analyzed by combining the characteristic of regional faults, the fault slip model of the Zadoi earthquake and the result calculated by boundary element method.

  Results  The main results are given as follows: (1) The ascending and descending InSAR coseismic deformation field is continuous and smooth as a whole, and the local surface deformation feature around the epicenter is obvious. The standard deviations for the ascending and descending observations are 4.3 mm and 3.4 mm, respectively. The maximum subsidences along the line of sight of ascending and descending orbits are -3.6 cm and-3.8 cm, respectively. The deformation on both sides of the earthquake fault is asymmetrical.(2)The fault slip does not extend to the surface, which is mainly concentrated at-3.80 km to -16.17 km depth, and there is one significant slip area. The maximum value of fault slip is about 0.27 m, which is located at-7.61 km to -8.56 km depth. The average slip angle is -36.04°. The seismic moment is 7.70×10¹⁷ N·m, and the corresponding moment magnitude is Mw 5.9, which is generally consistent with the existing results derived from seismic wave and InSAR observations. (3)The fault slip model can explain the observed coseismic deformation field better. The root mean squares of the fitting residuals for the ascending and descending observations are 4.4 mm and 3.7 mm, which are close to the standard deviations of 4.3 mm and 3.4 mm for the corresponding observations. (4) The fault slip of the Zadoi Mw 5.9 earthquake may be related to the rupture of the NEE trending fault under regional strain field, which is dominated by left-lateral strike-slip with a normal component.

  Conclusions  The obtained ascending and descending InSAR deformation field can accurately reflect the coseismic deformation characteristics of the Zadoi Mw 5.9 earthquake. The derived fault slip model can finely reveal the coseismic slip distribution below the earth's surface. The analysis of the earthquake fault slip with boundary element method can make up for the inadequacies of qualitative interpretation in the previous studies.The related results can provide references for the study on the regional crustal movement and deformation, active fault and earthquake preparation.