海原-六盘山断裂带现今地震危险性的数值模拟分析

Numerical simulation of the present seismic risk of the Haiyuan-Liupanshan fault zone

基于正交各向异性理论表征断层的变形行为,将平行断层面的剪切模量和周围介质剪切模量的比值作为反演参数,以海原-六盘山断裂附近现今GPS观测地壳水平运动速度场作为约束,通过构建三维有限元模型,采用遗传算法,反演了海原-六盘山断裂平行断层面的剪切模量分布。结果显示:六盘山断裂中南段平行断层面剪切模量与周围介质接近,且沿断层面地震动活动较为稀疏,反映六盘山断裂两侧近场差异变形较小,和汶川地震前龙门山断裂的情况类似,可能断裂带处于强闭锁状态。整个狭义的海原断裂带平行断层面剪切模量比周围介质要小的多,在0.4以下,且0~5 km要比深部大,可能反映了1925年海原8.5级地震之后,该断裂仍然处于震后调整状态。西段金强河断裂、毛毛山断裂、老虎山断裂浅部0~5 km剪切模量较小,而在5~20 km剪切模量相对较高,结合沿断层面地震活动分布特征,认为金强河、毛毛山断裂浅部可能存在蠕滑,而深部5~20 km存在应变能积累特征,具有强震发生的背景,而老虎山断裂由地表至深部地震活动较为密集,可能存在贯通性蠕滑,强震发生的可能性较小。

In this study,the orthotropic theory-based characterization of fault deformation behavior was made,with the ratio of the shear modulus parallel to fault plane to the shear modulus of surrounding media as the inversion parameters and the present-day crust horizontal movement velocity field observed by GPS near the Haiyuan-Liupanshan fault as the constrain.We built a 3D finite element model,using the genetic algorithm,to estimate the shear moduls distribution parallel to the Haiyuan-Liupanshan fault plane.The inversion results show that the shear modulus parallel to the Liupanshan fault plane is close to that of the surrounding media,and the seismic activities are sparsely distributed along the fault plane,reflecting a small deformation difference on both sides of the Liupanshan near the fault,which is similar to the situation of the Longmenshan fault before the Wenchuan Earthquake.The fault zone may be in a state of strong locking.The shear modulus parallel to the fault plane of the Hanyuan fault in the narrow sense is much smaller than that of the surrounding media,all below 0.4,and the shear modulus within 0~5 km is larger than that of the deep,which may reflect that the entire fault has still been in post-earthquake adjustment since the Haiyuan 8.5-magnitude earthquake in 1925.The shear modulus of the Jinqianghe,Maomaoshan and Laohushan faults in the western section is relatively low at the shallow section of the faults(0~5 km),while the shear modulus of 5~20 km is relatively high.Combined with the fault surface seismic activity distribution characteristics,it is considered that creep slips may exist in the shallow sections of the Jingqianghe and Maomaoshan faults;however,there is strain energy accumulation in the depth of 5~20 km,which has the background of strong earthquake.Seismic activity of the Laohushan fault is relatively intensive from the surface to the deep,and there may be creep through,with a small probability of strong earthquake.