In this study, 334 accelerograms of 42 small‐to‐moderate earthquakes recorded at 36 strong‐motion stations were used to investigate the ground‐motion characteristics of the southwestern margin of the Sichuan–Yunnan rhombic block (SWM‐SYRB), one of the most active seismic regions in China. The high‐frequency attenuation parameter was estimated using the spectral decay method, and the site component (κ0) was fitted. The κ0 estimates decrease with the increasing time‐averaged S‐wave velocity of the uppermost 30 m (VS30). Using the generalized inversion technique, the source spectra, quality factor (Q), and site amplification were derived from the Fourier amplitude spectra (FAS). The obtained average stress drop for earthquakes occurred in SWM‐SYRB was the second largest among various boundary areas of SYRB. The inverted Q model was Q(f)=115.1f0.616. The low Q structure that extends southwestward from the Songpan–Garze block to SWM‐SYRB could be responsible for the strong regional attenuation of ground motion with distance. At frequencies above 10 Hz, the average site amplifications were influenced by the high‐frequency attenuation effect. The site amplification of site class D reached a factor of 6 at 0.7 Hz. Moreover, it was observed that site amplification factors can be even higher when peak ground acceleration is larger than 0.8  m/s2. Finally, the obtained parameters were used in the stochastic finite‐fault simulation method to reproduce the FAS, 5%‐damped pseudospectral acceleration, and time series of the 2021 Mw 6.1 Yangbi earthquake. The simulated spectra properly matched the observations in a broad frequency band of 0.1–25 Hz. Furthermore, the simulated time series could generally represent the amplitude of the S‐wave portion of observed ones.

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