Abstract:

On December 18, 2023, a M_W6.1 earthquake struck Jishishan County in the Linxia Hui Autonomous Prefecture of Gansu Province. The strong ground shaking caused widespread building collapse and significant casualties, underscoring the severe societal impact of seismic events in this region. Because the epicenter is located within a tectonically active area characterized by frequent crustal deformation and complex fault interactions, clarifying the seismogenic mechanisms is essential for understanding regional seismic hazards.

To investigate the fault structures and rupture processes associated with the event, we applied time-series Interferometric Synthetic Aperture Radar(InSAR) analysis to descending-orbit deformation rate fields across the epicentral region, integrating these results with pre-earthquake Global Positioning System(GPS)observations. Constrained by these geodetic datasets, fault coupling behavior was estimated using a Markov Chain Monte Carlo(MCMC) inversion. The preferred fault model suggests an interseismic slip rate of ~1.9mm/a and a fault locking depth of ~11km. These results are consistent with geological field observations, providing validation for the geodetic modeling. The inversion also reveals a crustal shortening rate of ~1.0mm/a across the Lajishan Fault, highlighting the potential for reverse-faulting earthquakes in the region.

For coseismic deformation, we employed both ascending- and descending-track Sentinel-1 SAR acquisitions to extract high-resolution displacement fields. Using these InSAR constraints, the optimal fault geometry was determined from posterior probability density distributions of fault parameters. A linear inversion incorporating Laplacian smoothing was then applied to estimate the coseismic fault slip distribution. The results show that the earthquake was generated by low-angle thrust faulting, with the seismogenic fault plane striking 131° and dipping southwest at 23.3°. A prominent high-slip asperity was identified at depths of 3~8km, with a maximum slip of 0.35m at ~6.8km depth, consistent with regional reverse-faulting characteristics. Model residuals are mostly <5mm, concentrated northwest of the epicenter, where decorrelation is relatively strong.

Coulomb stress changes were further calculated based on the inverted slip model. The analysis shows that coseismic slip significantly modified stress distributions, with most aftershocks occurring in regions of increased Coulomb stress. This demonstrates the critical role of post-seismic stress redistribution in controlling aftershock occurrence and highlights the importance of stress analysis for seismic hazard assessment.

Beyond the main rupture, the earthquake also affected adjacent fault systems, including the middle-northern segment of the Jishishan Fault and the central segment of the northern margin fault of the Western Qinling Mountains. These segments now experience elevated Coulomb stress, implying heightened seismic risk. This finding underscores the necessity of continuous geodetic monitoring and regular seismic hazard reassessment in the region.

By integrating GPS and InSAR observations, this study provides a comprehensive and quantitative analysis of coseismic deformation and fault motion during the Jishishan earthquake. The results not only enhance understanding of earthquake dynamics but also contribute to improved seismic hazard assessment and long-term risk mitigation in tectonically active regions.

Key words: Jishishan Earthquake, interseismic fault motion, InSAR coseismic deformation, fault slip model, Coulomb failure stress

摘要： 2023年12月18日午夜, 中国甘肃省临夏市积石山县发生 M_W6.1 地震, 强烈的同震晃动造成大量房屋倒塌及人员伤亡。为揭示此次地震的发震构造和震前、 同震运动机制, 文中利用震前GPS和时序InSAR形变, 反演得到孕震断裂震间走滑速率约为1.9mm/a, 断层闭锁深度约为11km, 跨断裂地壳缩短率约为1.0mm/a, 具备发生逆冲地震的潜力。进一步利用Sentinel-1卫星获取的地震前后的合成孔径雷达数据, 提取了同震InSAR形变场, 根据搜索断层参数后验概率密度分布, 估算了最优断层几何参数, 并反演了同震断层滑动模型。结果表明, 积石山地震为典型的低倾角逆冲断层运动, 断层倾向SW具有更高的置信度, 最大滑动量位于地下约6.8km深度处, 约为0.35m。此外, 不同深度处的库仑应力变化显示, 主震断层滑动造成的库仑应力传输显著促进了余震的发生。积石山南缘断裂的中北段、 积石山北缘断裂东南段及西秦岭北缘断裂的中段均出现明显的库仑应力增加, 需持续关注这些区段的地震风险。

关键词: 积石山地震, 震间断裂运动, InSAR同震形变, 断层滑动模型, 库仑应力