Regional ground motion characteristics and topographic effect in the 2023 December Ms 6.2 Jishishan earthquake and its implication for ground motion model development in Northwestern China

Abstract

Ground motion model (GMM) is an important component of seismic hazard analysis in Northwestern China. The Ms 6.2 Jishishan earthquake in the Lajishan Mountain Fault zone of Northwestern China was well recorded by the densely distributed China Seismic Intensity Rapid Reporting and Earthquake Early Warning System, providing a good basis for GMM development of this region. In this study, we evaluate the applicability of five high-quality GMMs in this seismic zone, including two developed specifically for Southwest China, two from the NGA-West2 project, and one for Japan, using the records from this earthquake. We found that the NGA-West2 models and the Japan model outperform those developed for Southwest China, which exhibit significant underestimation of the attenuation effect and site effect. We examine the regional differences in ground motion attenuation and site response between the Loess region and the nonLoess region. Our findings reveal that the Loess region has significantly slower attenuation rates and stronger V_S30 scaling rates than the nonLoess region at short spectral periods up to about 0.6s. Accounting for these regional differences could substantially reduce ground motion modeling errors. Additionally, a clear correlation between the ground motion residual and the topographic position index (TPI) is identified at periods longer than 0.6s. We interpret it as regional topographic effect and it is probably affected by both the surface topography and the large-scale subsurface structure. Accounting for this effect could further lead to a decrease in model variability. The results are helpful for the improvement of GMMs for Northwestern China or specifically for the Lajishan Mountain Fault zone.