Regional‑Scale Full 3‑D Hybrid Waveform Forward Modeling

Accurate velocity models are essential for fitting high‑frequency seismic waveforms, yet regional‑scale 3‑D simulations are computationally expensive, especially with sparse seismic networks. We propose a hybrid waveform simulation approach that combines the 3‑D spectral‑element method (SEM) with the displacement representation theorem. By separating near‑source propagation from long‑distance propagation to stations, only the near‑source wavefield needs to be recomputed when the local velocity and source models change. We apply the method to the 2019 Mw 5.0 Changning shallow earthquake to verify its effectiveness. We compare high‑frequency waveforms computed with different regional velocity models against observations. Results show that the hybrid method achieves accuracy comparable to full SEM simulations while reducing computation costs by over two orders of magnitude when the number of source-region updates greatly exceeds that of stations. Our results further indicate that high‑frequency waveforms are highly sensitive to shallow structures. Introducing low‑velocity shallow layers into the source region improves near‑field waveform fits, indicating pronounced low‑velocity sediments in the Changning area. Large surface wave time delays suggest that shallow velocities within the Sichuan Basin are lower than those in existing models. In addition, an InSAR‑derived finite‑fault model outperforms the point‑source model in near‑field waveform fitting and better reproduces rupture directivity. The proposed method is practical for high‑frequency waveform modeling in areas with complex subsurface structures.