Near-fault broadband ground-motion simulation of 2021 Yangbi M6.4 earthquake: an improved FK method

Extending the physics-based ground-motion simulation from the existing 1~2 Hz resolution to the higher frequency of 5~10 Hz, which is sensitive to the engineering structure, is a critical development direction of near-fault ground-motion simulation in modern seismic engineering. An improved frequency-wavenumber domain (FK) method is established, combined with the GP14.3 hybrid-source model, to achieve an efficient simulation of 0~10 Hz near-fault ground motion. A revised stiffness matrix method is established to solve the theoretical Green's function, with which the problem of propagation of high-frequency seismic waves in the 1D velocity crustal structure is effectively solved. The reasonable combination of low-frequency deterministic parts and high-frequency stochastic parts on the finite-fault plane effectively solves the problem of high-frequency seismic waves radiated from the rupture process. The method is applied to the simulation of the M6.4 shallow damage earthquake in Yangbi of Yunnan on May 21, 2021. The comparison with the strong-earthquake records at eight stations (covering the near-field, mid-field, and far-field) and the response spectra shows that the simulated results are in good agreement with the waveform, duration and amplitude of the records, and agree with the response spectra in each frequency band, which well verifies the applicability of the proposed method and model and the reliability of the simulated frequency bandwidth. Finally, the ground motions within the range of 100 km×100 km at the regional scale are simulated, and the PGA distribution and velocity wavefield snapshots in the Yangbi area are given. The PGA and PGV empirical attenuation equations and the spectral characteristics are proposed. The results show that: (1) The 2021 Yangbi M6.4 earthquake exhibits obvious concentration effects and rupture directional effects. (2) The peak ground motion attenuation is faster within 20 km of the near field, that is, the maximum attenuation of PGA is 93.1%, and the maximum attenuation of PGV is 83.3%. (3) The frequency components in the near-field range mainly include 0~10 Hz broadband components. When the epicenter distance exceeds 20 km, the frequency components are concentrated primarily in a range of 0~4 Hz.