Probabilistic seismic hazard analysis for the northern segment of the North-South Seismic Belt in China based on improved spatial smoothing and fault source model integration

The northern segment of the North-South Seismic Belt is characterized by intense crustal deformation, well-developed active tectonics, and frequent occurrences of strong earthquakes. Therefore, conducting a Probabilistic Seismic Hazard Analysis (PSHA) for this region is of significant importance for supporting seismic fortification in major engineering projects and formulating disaster prevention and mitigation policies. In this study, a composite seismic source model was constructed by integrating data on historical earthquakes, active faults, and paleoseismicity. Furthermore, a logic tree framework was employed to quantify epistemic uncertainties, enabling a systematic seismic hazard assessment of the region. To more accurately characterize the spatial heterogeneity of seismic activity, improvements were made to both the Circular Spatial Smoothing Model (CSSM) with a fixed radius and the Adaptive Spatial Smoothing Model (ASSM), with full consideration given to the spatiotemporal completeness of historical earthquake magnitudes. Regarding the CSSM, for scenarios involving small sample sizes in earthquake catalogs, the cross-validation method proposed in this study demonstrated higher robustness than the maximum likelihood method in determining the optimal correlation distance. Performance evaluation results indicate that while both models effectively characterize seismic activity, the ASSM exhibits superior overall predictive performance compared to the CSSM, owing to its ability to adaptively adjust the smoothing radius according to seismic density. Significant discrepancies were observed in the Peak Ground Acceleration (PGA) results calculated with a 10% probability of exceedance in 50 years across different combinations of seismic source models. The single spatially smoothed point-source model yielded a maximum PGA of approximately 0.52 g, with high-value areas concentrated near historical epicenters, thereby significantly underestimating the hazard associated with major fault zones. When combined with the simple fault-source model, the maximum PGA increased to 0.8 g, with high-value zones exhibiting a zonal distribution along faults; however, the risk remained underestimated for faults with low slip rates that are nevertheless approaching their recurrence cycles. Following the introduction of the time-dependent characteristic fault-source model, local PGA values for faults in the middle-to-late stages of their recurrence cycles increased by a factor of 2 to 7 compared to the single model. These results demonstrate that the characteristic fault-source model reasonably delineates the time-dependence of large earthquake recurrence, thereby providing a more accurate assessment of imminent seismic risks. By comprehensively applying the improved spatially smoothed point-source model, the simple fault-source model, and the characteristic fault-source model, the following faults within the region were identified as having high seismic hazard: the Huangxianggou, Zhangxian, and Tianshui segments of the Xiqinling northern edge fault; the Maqin-Maqu segment of the Dongkunlun fault; the Longriqu fault; the Maoergai fault; the Elashan fault; the Riyueshan fault; the eastern segment of the Lenglongling fault; the Maxianshan segment of the Maxianshan northern Margin fault; and the Maomaoshan-Jinqianghe segment of the Laohushan-Maomaoshan fault. As these faults are located within seismic gaps or are approaching the recurrence periods of large earthquakes, they should be prioritized for current and future seismic monitoring as well as disaster prevention and mitigation efforts.