Ground-Motion Simulation and Surface Topography Effects of the 2022 MS 6.8 Luding, Southwest China, Earthquake

A strong earthquake of magnitude (M_S) 6.8 has struck Luding county in Sichuan province, southwestern China, on 5 September 2022 at 04:52:18 UTC. The Luding Earthquake occurred at the junction of the eastern edge of the Qinghai-Tibet Plateau and the Sichuan Basin. The affected area features highly rugged terrain with an elevation difference of nearly 7 km, providing an opportunity to study the topographic effects on seismic ground motion. In this study, a flat surface model (3DFlat model) and a model incorporating surface topography (3DTopo model) were developed. The low-frequency part of the ground motion is simulated using a curvilinear grid finite difference method, while the high-frequency part is simulated using a three-component stochastic finite fault model. The low- and high-frequency results are combined to synthesize broadband ground motion.

The results show that the scattering effects caused by the dramatic topographic relief complicate the wavefields of the 3DTopo model and the overall match with the waveform and spectral characteristics of the observation records. The 3DTopo model has a richer high-frequency component compared to the 3DFlat model, while the ground motion below 0.1 Hz is not affected by surface topography. Comparing the 3DFlat and 3DTopo models reveals that the multiple scattering effects of seismic waves caused by ridge and canyon topography result in irregular wavefront shapes, with numerous scattered and reflected waves in the velocity waveforms. The distribution of the peak parameters ln(δPGA) and ln(δPGV) shows significant correlations with surface topography. The distribution of amplification (attenuation) of ground motion corresponds to the orientation of mountain ridges and valleys. Ground motion is significantly amplified at wave crests and ridges (ln(δPGA) > 0), with the amplification of PGA and PGV reaching up to 5.4 times and 3.6 times, respectively. In contrast, ground motion is significantly attenuated in valleys (ln(δPGA) < 0), with PGA and PGV reduced by up to 0.40 times and 0.45 times, respectively. Our further research on the relationship between ground motion and topographic features establishes a correlation between the topographic amplification factor AF_TOPO and the Relief Degree of Land Surface (RDLS).

In addition, we also used a frequency-domain matching technique to combine low- and high-frequency results into broadband ground motion. Comparisons with observed records and four NGA-West2 ground motion models (ASK14, BSSA14, CB14, and CY14) show that, although the residuals of ground motion parameters (PGV, PGA, PSA) obtained by different methods fluctuate with the period. This study will be an important to promote the incorporation of topographic effects into seismic zoning.