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

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