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 limited by the distribution of stations, the relatively fine three-dimensional crustal velocity structure is still lacking. The middle section of North Tianshan is the most concentrated area of population and economy in Xinjiang. There are a series of active faults in the piedmont, causing frequent occurrence of strong earthquake in the region. Due to the lack of targeted research on the deep structure of the region, it is difficult to understand the mechanism of earthquakes in the region. In this study, we used the observations of 14 fixed stations and 4 temporary stations set up in the middle section of north Tianshan Mountains in Xinjiang to obtain the three-dimensional velocity structure of the crustal by using local seismic tomography and relocated the local seismic events in the area using the three-dimensional velocity structure. The seismic events used in the inversion were recorded by at least 5 stations, and the azimuth coverage was greater than 180°. Finally, 629 seismic events were obtained, including 5 238 P-wave traveltimes and 2 144 S-wave traveltimes. The time-distance curve indicates that there are some seismic events with deeper focal depths in the study area. After relocation, the residual is stable at about 0.52, the theoretical deviation of the source position is about 0.5km, the root mean square of the residual is reduced from 0.8 seconds to 0.5 seconds, and the positioning accuracy is obviously improved. 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. At a depth of 5km, the shallow layer shows high-velocity zone beneath Tianshan and low-velocity zone on the side of the Junggar Basin, which is characterized by a high-velocity anomaly along the Tianshan Mountains belt. At a depth of 10km, the study area is dominated by high-velocity anomalies. Among them, with Hutubi as the boundary, the east part is characterized by high-velocity anomalies, and the west is the low-velocity anomalies, showing an obvious velocity interface. At a depth of 15km, the east of Hutubi shows a relatively low-velocity anomaly, and the west shows a high-velocity anomaly, indicating that there is a large gradient of P-wave velocity in this range. At a depth of 20km, the area shows low-velocity anomalies, but on the southern Junggar fault zone, it shows high-velocity anomalies. In the middle and lower crust, the area is dominated by low-velocity anomalies near the southern margin of Junggar and the Bogda arc fault. The low-velocity zone may be caused by a large ductile shear system in the area. S-wave is limited by observation conditions, and the upper and middle crust exhibit characteristics such as relatively low wave velocity and low wave velocity ratio. 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, and structural deformation is strong. There is a seismogenic zone extending about 10km underground from the surface to the middle and lower crust around the Hutubi gas storage. The seismic activity of this area is importantly related to the gas injection and pumping of the gas storage. In addition, the relocation distribution characteristics of the aftershocks of Hutubi earthquake 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-dipping, and the dip angle is about 50°. Aftershocks are basically located above the main shock, and the initial rupture point is at the bottom, causing rupture from bottom to top. At the same time, the source is located in a high-velocity anomaly area, which creates conditions for the preparation and occurrence earthquake.

Key words: the middle section of north Tianshan Mountains 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受到研究者的高度关注。前人对天山地区的深部结构开展了诸多研究, 但受限于地震台站的分布, 较为精细的地壳三维速度结构结果尚比较缺乏。文中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10a的观测数据及4个流动台站的观测资料进行了近震走时层析成像, 获得了地壳的三维速度结构, 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, 新疆天山中段的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, 区内在昌吉附近呈现近SN向的高速异常, 在准噶尔南缘断裂带附近也表现为高速异常; 在中下地壳, 区内以准噶尔南缘及博格达弧形断裂附近的低速异常为主, 该低速带可能是区域内大型韧性剪切系统所致。地震重定位结果显示, 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的地温梯度较低, 同时该区域的波速比和P波波速均相对较低, 构造变形强烈。此外, 2016年呼图壁 M_S6.2 地震的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S倾, 倾角约为50°; 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