Finite fault inversion is an effective method commonly used to study the rupture process of earthquakes and has been widely applied in seismic source rupture process research. In this study, we utilized the method to investigate the rupture process of the MS
6.9 Menyuan, Qinghai earthquake in 2022 using regional broadband seismic waveform data. Considering the quality and distribution of the network data, we selected 20 regional seismic stations with high signal-to-noise ratio and epicentral distances less than 500 km. We preprocessed the observed waveforms by removing instrument response, demeaning, detrending, and filtering. We also calculated the near-field Green’s functions using the frequency-wavenumber （f
） method. First, synthesize the waveforms using the Green’s function synthesis theory and compare them with corresponding observed waveforms. Then, perform fault model inversion where the displacement response of each sub-fault at any station can be represented as a function of the sub-fault’s slip, direction, rise time, and rupture velocity. Additionally, in the process of reducing errors, we use wavelet transform to establish the objective function and use simulated annealing （SA） to search for the global optimal solution of each sub-fault parameter, so that the waveform residual can be minimized in the wavelet domain, ultimately obtaining the best spatio-temporal distribution model of the source fault slip. In this study, high frequency signal of regional network is used. Low-frequency information constrains the overall characteristics of the earthquake source, but is less sensitive to detailed rupture features such as rupture rise time or rupture velocity changes. However, sudden changes in slip amplitude or rupture velocity will radiate intense seismic signals with higher-frequency. Therefore, using higher-frequency signals for finite fault inversion can effectively improve spatial and temporal resolution, and fully utilize the wide-band information of seismic waves to better understand the earthquake source rupture process. The following results were obtained: ① The seismic fault is nodal planeⅠ（strike 104.2°, dip 84.6°, rake −5.4°）. The rupture is mainly concentrated in two areas. The first area is a circular region with a radius of approximately 3 km above the hypocenter, with a maximum displacement of about 1.5 m, located at a depth of about 6 km underground, where the depth of obvious rupture is about 14 km. The earthquake activity ruptures to the surface with a maximum displacement of approximately 0.5 m, and the surface rupture length is about 20 km. Second, along the strike direction, the circular area with a radius of about 3 km is at the lower-right of the hypocenter, with a maximum displacement of approximately 1.2 m, at a depth of about 14 km. ② The scalar seismic moment released by the earthquake is 1.23×1019
N·m, corresponding to MW
6.7. The earthquake rupture lasts about 17 s, with the maximum release of energy occurring around the 8 s. The energy was mostly released before 15 seconds. ③ From the perspective of rupture direction, the rupture mainly propagates along the ESE direction. The maximum rupture values appear on both sides of the epicenter at 2 s and 9 s, reflecting the characteristics of bilateral rupture. According to the surface dislocation distribution map, the inclination of the seismic fault surface is 84.6°, which is close to vertical. Therefore, the surface projection with obvious dislocation distribution is as long as 34 km. ④ Two obvious features can be identified: first, the fault plane has a large inclination angle and releases a high amount of energy, resulting in significant surface ruptures with a length of approximately 18 km; second, there is bilateral rupture, as evident from the large displacements observed on both sides of the epicenter at 2 s and 9 s. The regional tectonic position also confirms that the initial rupture occurred along the Tuolaishan fault in an approximate E-W direction and triggered the NW-SE trending Lenglongling fault, which is closely related to the complex tectonic environment in the area.environment in the area.