Numerical simulation of the present seismic risk of the HaiyuanLiupanshan fault zone
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摘要: 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1920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。Abstract: 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 1920. 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.
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Key words:
- Haiyuan-Liupanshan fault /
- GPS /
- finite element /
- orthotropy /
- genetic algorithm (ga)
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图 1 研究区构造特征及历史强震
Figure 1. Tectonic characteristics and historical strong earthquakes in the study area
图 3 研究区GPS观测速度场(1999—2016年)
Figure 3. GPS velocity field (1999-2016) in the study area
图 5 模拟速度场和GPS观测结果的对比
Figure 5. Comparison of the simulated velocity field and the GPS observation results
图 6 残差及其统计分析
图中柱状灰度深浅代表文中公式(4)中δi的大小
Figure 6. Residual and statistical analysis. The gray shade of the column represents the magnitude of δi in Equation (4)
图 7 模拟结果和观测结果分别计算的应变率场
图中色卡对应颜色表示最大剪应变率大小，十字叉表示主压、主张应变率大小和方向
a—观测结果计算的主应变率和最大剪应变率场；b—模拟结果计算的主应变率和最大剪应变率Figure 7. Strain rate fields calculated using simulated and observed results respectively. The colors indicate the value of the maximum shear strain rate, and the crosses indicate the size and direction of the principal compressive and tensile strain rate. (a) Principal strain rate and maximum shear strain rate field calculated from the observation results. (b) Principal strain rate and maximum shear strain rate calculated from the simulation results
图 8 反演得到的断层剪切模量与周围介质剪切模量的比值(0~1之间)
Figure 8. Ratio of the shear modulus of the fault obtained by inversion to the shear modulus of the surrounding media (between 0 and 1). The color blocks represent the ratio of the shear modulus of the parallel fault plane of each block to the shear modulus of the surrounding medium. The white circles represent the projection on the fault plane of the precise positioning results of small earthquakes (magnitude 3.0 or above) within 10 km from each side of the fault
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