关键词: 弹性块体模型, GPS, 天山地区, 滑动速率, 地震矩亏损

Abstract: Late Cenozoic and modern tectonic deformation in mainland China is mainly characterized by active block movement, and the average slip rate of faults in the fault zone at the block boundary is an important indicator for quantitatively measuring the intensity of fault activity. The Tianshan Mountains, as the largest revival orogenic belt within Eurasia, with crustal movement basically manifesting as near north-south deformation and a large number of strong seismic surface ruptures, is one of the regions with strong tectonic movement and one of the key seismic hazard zones in China. Many experts have conducted relevant studies on the Tianshan region using GPS technology and have obtained some useful conclusions. These studies have not divided and analyzed the fault zone in detail, but only divided the Tianshan seismic zone into several major fault zones, such as the eastern and western sections of the northern Tianshan, and the eastern and western sections of the southern Tianshan. In order to analyze the activity characteristics of the major faults in the Tianshan region more clearly, this paper refines the major faults and selects 14 major active faults in combination with the distribution of active faults in China proposed by Xu Xi-wei et al. 18 blocks are divided into secondary blocks in Tianshan region, with the major active blocks in the Tianshan region taken as the boundary; The GNSS data of the surrounding areas of 1999—2015 in the Tianshan seismic zone are collected in this paper and used to calculate the velocity field results, and the block locking depth and the slip rate of major faults are calculated using the elastic block model to quantify the seismogenic capacity of major faults. Because the fault closure will produce obvious elastic deformation gradient around the fault, the greater the depth of fault closure is, the greater the influence will be. The fault locking depth can be constrained by the method of GPS data fitting of this model, and the influence of fault locking depth is verified by the method of GPS minimum residual RMS in this paper. According to the optimal locking depth obtained in this paper, the velocity field in Tianshan area is simulated and calculated. The residual mean value of the velocity field simulated by the elastic block model is small, and the average velocity error in the east-west direction is 1.57mm/a, the average velocity error in the north-south direction is 1.72mm/a. At the same time, the slip rate of major faults is obtained. The results show that: the horizontal shortening of the whole Tianshan region is significant, which is consistent with the tectonic background of the region, and the shortening value in the southern Tianshan region is higher than that in the northern Tianshan region; the shortening tensile rate is significantly larger than the slip rate, which shows that the fault zone at basin mountain junction in the Xinjiang Tianshan region is dominated by backwash activity; the extrusion rate in the western section of the southern Tianshan fault zone is in a high value state, reaching(-6.3±1.9)mm/a, which is higher than that in the eastern part of the southern Tianshan; the extrusion rate in the western part of the northern Tianshan is also higher than that in the eastern part. All the strong earthquakes of magnitude 8 and more than 80% of the strong earthquakes of magnitude 7 and above in China occurred in the boundary zones of active blocks according to the historical records, the motion characteristics of the boundary zone of active blocks play an important role in controlling the generation and occurrence of earthquakes, and the seismicity of faults may be quantitatively calculated by the loss of seismic moment. In this paper, we collected a list of strong earthquakes of magnitude 6 and above in the Tianshan area since 1900, estimated the seismic moment release of the main faults in the Tianshan seismic zone based on the above list, and compared it with the calculated seismic moment accumulation to obtain the seismic moment loss of the corresponding fault. Among them, the maximum release of seismic moment of the Beiluntai Fault reached 8.69×10¹⁹N·m; due to the release of several moderate and strong earthquakes, the seismic moment of middle of Bo-A Fault and Keping Fault have not reached the deficit state at present, the surplus is -1.85×10¹⁹N·m and -3.06×10¹⁹N·m, respectively; The smallest area of earthquake release is the northern Tianshan mountain front fault, which is only 0.11×10¹⁹N·m, because there was only one earthquake with a magnitude of 6 in 1907, and the earthquake accumulation reached 11.53×10¹⁹N·m, generating an earthquake deficit of 11.42×10¹⁹N·m, which could produce a magnitude of 7.3 earthquake. The results show that front margins of the northern Tianshan Fault, the Maidan Fault, the north section of Ertix Fault and the west of Kashihe Fault have a large seismic moment loss and have the potential to generate earthquakes of magnitude 7 and above, while Beiluntai Fault and the middle section of the Keping Fault show a surplus state, and there is no possibility of a strong earthquake in a certain period of time in the future.

Key words: elastic blocks, GPS, Tianshan region, slip rate seismic moment deficit