Investigating ground motion anomalies in the 2023 M w 5.9 Jishishan, China, earthquake: A physics-based simulation perspective

The M w 5.9 Jishishan earthquake, China, is a thrust-type event that occurred on 18 December 2023, causing extensive damage to buildings and geological features in the epicentral area. Post-earthquake damage surveys and historical earthquake data indicate that thrust faulting and sedimentary effect are key contributors to the observed damage patterns. To assess how these factors influence ground motion, broadband simulations are conducted within the 0–5 Hz frequency range, employing a physics-based deterministic approach to model earthquake rupture and wave propagation. The numerical model is first validated by comparing data from 20 well-recorded stations during the Jishishan earthquake. Subsequently, three different models (Topography, Flat, and Bedrock) were constructed to assess the impact of thrust fault and sediment on ground motion. The results show that the ground motion on the hanging wall is much higher than that on the footwall, demonstrating a pronounced hanging wall-footwall effect, with a maximum hanging wall-to-footwall ratio of 3.62. Furthermore, the simulated ground motions with and without sedimentary structures indicate that sediment tends to focus the seismic wavefield, resulting in amplification and prolongation of the ground motion, with peak ground motion and duration amplifications reaching 2.57 and 2.15 times, respectively. These findings suggest that the coupling effect of thrust fault and sedimentary structure dictates the ground motion distribution of the Jishishan earthquake. The identified hanging wall-footwall effect and sedimentary amplification provide a reasonable explanation for regional ground motion anomalies.