Abstract Around the southern front of the Qilian Mountains, three Mw 6.3 strong earthquakes have occurred since 2003. The distances between the epicenters of these events range from 18 to 46 km. By collecting available datasets, including broadband waveforms, coseismic surface geodetic measurements and active fault mapping results, we invert rupture models for these strong earthquakes with a finite fault model approach in elastic half space. Constrained by these detailed rupture models, we then build a three-dimensional finite element model to calculate Coulomb stress change due to the coseismic displacement and postseismic viscoelastic relaxation. Combined with an effective friction coefficient of 0.2–0.8 and variable rheological characteristics, numerical results show that following the 2003 event, the Coulomb stress change along the 2008 rupture segment was positive with magnitudes between 1.8 × 103 Pa and 2.2 × 103 Pa. Among the contributing stresses, the postseismic viscoelastic relaxation contributed 10.7% ~ 23.3%. Just prior to the occurrence of the 2009 earthquake, the total changes of the Coulomb stress around its hypocenter reached 2.71 × 104– 3.72 × 104 Pa, to which the 2003 event and the 2008 event contributed approximately 7 × 102– 1.5 × 103 Pa (2.58% ~ 4.03%) and 2.64 × 104– 3.57 × 104 Pa (95.97% ~ 97.42%), respectively. Other postseismic behaviors, including afterslip and poroelastic rebound, can also yield extra changes in the Coulomb stress, which probably enhance the conclusions that the 2003 earthquake likely advanced the occurrence of the 2008 event by several years and that the 2008 earthquake likely triggered the 2009 earthquake directly. The numerical results also show that the increased stress around the hypocenters of latter two events was mainly determined by the positive shear stress. After the three strong earthquakes, the Coulomb stress on some active faults increased and is still increasing, suggesting that great attention should be paid to the potential earthquake hazards in this region, especially along the Zongwulongshan fault.

中文翻译：

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青藏高原东北部祁连山南缘3次Mw～6.3级地震破裂模型及粘弹性应力变化

摘要 祁连山南缘自2003年以来共发生3次6.3级强地震，震中距18～46公里。通过收集可用的数据集，包括宽带波形、同震地表大地测量和活动断层测绘结果，我们用弹性半空间中的有限断层模型方法反演这些强地震的破裂模型。受这些详细的破裂模型的约束，我们然后建立了一个三维有限元模型来计算由于同震位移和震后粘弹性松弛引起的库仑应力变化。结合 0.2-0.8 的有效摩擦系数和可变的流变特性，数值结果表明，在 2003 年的事件之后，2008年破裂段库仑应力变化为正，震级在1.8×103Pa～2.2×103Pa之间。在贡献应力中，震后粘弹性松弛贡献了10.7%～23.3%。就在2009年地震发生前，其震源附近库仑应力的总变化达到2.71×104～3.72×104 Pa，其中2003年和2008年事件贡献了约7×102～1.5×103 Pa（ 2.58% ~ 4.03%) 和 2.64 × 104– 3.57 × 104 Pa (95.97% ~ 97.42%)。其他震后行为，包括后滑和多孔弹性回弹，也会导致库仑应力发生额外变化，这可能加强了 2003 年地震可能将 2008 年地震的发生提前几年以及 2008 年地震可能直接引发 2009 年地震的结论。数值结果还表明，后两个事件震源周围应力的增加主要是由正切应力决定的。3次强震后，部分活动断层库仑应力增大，且仍在增大，表明该地区尤其是宗五龙山断裂带的潜在地震危害应引起高度重视。