Abstract: As one of the largest and most active intraplate orogenic belts in the world, Tianshan has experienced significant crustal shortening and uplift since the Neogene. Its crustal deformation and structure have been deeply concerned by researchers. The predecessors carried out many studies on the deep structure of the Tianshan area, but it is limited by the distribution of stations, and the relatively fine three-dimensional crustal velocity structure is still lacking. In this study, we used 14 fixed stations and 4 temporary stations set up in the middle section of north Tianshan mountain in Xinjiang. We obtained the three-dimensional velocity structure of the crustal by using local seismic tomography, relocate the local seismic events in the area using the three-dimensional velocity structure. The inversion of the three-dimensional velocity structure shows that the velocity structure in the middle section of the Tianshan Mountains in Xinjiang shows obvious longitudinal inhomogeneity, which is a high-speed belt in the shallow Tianshan orogenic belt and a low-speed belt on the side of the Junggar Basin. At a depth of 10km, the study area is dominated by high-speed anomalies. At a depth of 20km, the area shows low-speed anomalies, but on the southern Junggar fault zone, it shows high-speed anomalies. In the middle and lower crust, the area is dominated by low-velocity anomalies in the east-west direction along the Tianshan orogenic belt, The low-velocity zone may be caused by a large ductile shear system in the area. The relocation results show that in the basin-mountain junction, especially near the Bogda arc structure, the middle and lower crust earthquakes occur frequently. The deeper depth of the source indicates that the area has a lower geothermal gradient, and the velocity structure in the area is lower than normal. The ratio of wave velocity also show lower than normal velocity, and structural deformation is strong. In addition, the relocation distribution characteristics of the Hutubi earthquake aftershocks show that the Hutubi earthquake occurred on the southern marginal fault zone of Junggar. The distribution of aftershocks indicates that the fault may be south-sloping, and the dip angle is about 50°. At the same time, the source is located in a high-speed anomaly area, which creates conditions for the occurrence.

Key words: The middle section of north Tianshan mountain in Xinjiang, Local seismic tomography, Velocity structure, Hutubi earthquake

摘要： 天山作为世界上最大、最活跃的板内造山带之一，在新近纪以来经历了显著的地壳缩短和隆升，其地壳变形和结构一直深受研究者的高度关注。前人对天山地区深部结构开展诸多研究，但受限于地震台站的分布，较为精细的三维地壳速度结构尚比较缺乏。本研究利用新疆测震台网在新疆北天山中段架设的14个固定宽频带地震台站近十年的观测数据及4个流动台站的观测资料，开展近震走时层析成像获得了地壳三维速度结构，并利用三维速度结构对该区域的近震事件进行了重新定位。反演结果显示，新疆天山中段速度结构呈现出明显的纵向不均匀性，天山浅层为高速带，准噶尔盆地一侧为低速带；在10km深度处，研究区基本以高速异常为主；在20km深度处，区内表现为低速异常，但在准噶尔南缘断裂带上，表现为高速异常；在中下地壳，区内以沿着天山造山带展布近东西向的低速异常为主，该低速带可能是区域内大型韧性剪切系统导致的。重定位结果显示，在盆山结合处，特别是博格达弧形构造附近，中下地壳地震时有发生，震源深度较大的特征指示该区域地温梯度较低，同时该区域速度结构比正常P波波速和波速比低，构造变形强烈。此外，呼图壁地震余震重定位分布特征显示，呼图壁地震发生在准噶尔南缘断裂带上，余震分布形态指示该断裂可能为南倾，倾角大约为50°左右，同时震源位于高速异常区，这为地震的孕育和发生创造了条件。

关键词: 新疆北天山中段, 近震层析成像, 速度结构, 呼图壁地震