Abstract:

As one of the most seismically active regions in China, the southwestern Tianshan exhibits the necessary tectonic conditions and energy accumulation for earthquakes exceeding magnitude 6. Investigating the region's tectonic deformation characteristics is crucial for understanding the background of strong earthquakes and assessing future seismic hazards.
In this study, we compiled and analyzed multi-period GNSS velocity fields from domestic and international sources, generating a fused velocity field. The results indicate that the crustal deformation of the Tianshan seismic belt is spatially heterogeneous, influenced by the combined effects of the clockwise rotation of the Tarim Basin and the northward thrust of the Pamir Plateau. Additionally, three north-south profiles across the study area were analyzed. The western Tianshan(Profile 1)exhibits a higher north-south compression rate, which gradually decreases eastward, consistent with the velocity field trends.
Using the fused GNSS velocity field, we calculated the strain rate field through the least-squares collocation method, obtaining strain characteristics of the Keping thrust belt and surrounding areas. The Piqiang fault zone serves as a boundary, with compressive strain on the western side of the Keping thrust belt exceeding that on the eastern side. The average surface strain rate is -3.3×10^-8/a in the west and -1.2×10^-8/a in the east. Similarly, the maximum shear strain follows this pattern, with values of 2.7×10^-8/a in the west and 1.1×10^-8/a in the east, indicating distinct deformation characteristics on either side of the Keping thrust belt.
Focal mechanism solutions of earthquakes(M≥4.0)since 1976 were collected, and stress inversion analysis was conducted at various depths. Earthquakes in the Tianshan region predominantly occur within the upper 50km of the crust. The results reveal a nearly north-south principal compressive stress orientation, perpendicular to the Tianshan orogenic belt, aligning with the regional surface principal compressive strain rate. However, due to the presence of conjugate strike-slip faults and strike-slip earthquakes in the northern and southern Tianshan hinterland, local horizontal principal stress directions deviate from the regional trend.
Integrating InSAR-derived deformation data with GNSS velocity field results, we calculated fault slip deficit rates and slip rates for major faults in the Keping thrust belt using a three-dimensional block model. The western Keping fault zone exhibits a higher compression rate of(3.1±0.3)mm/a, whereas the eastern section has a lower rate of(0.3±0.2)mm/a. The Maidan fault's western section has a compression rate of(2.7±0.5)mm/a, increasing to(3.7±0.4)mm/a in the east. Additionally, the left-lateral strike-slip rate is higher in the western Keping fault zone((1.5±0.3)mm/a)compared to the eastern section((0.5±0.2)mm/a). The Maidan fault follows a similar pattern, with strike-slip rates of(0.9±0.5)mm/a in the west and(2.1±0.4)mm/a in the east. The slip deficit rate distribution indicates high values in the western Keping fault zone, corresponding to the Jiashi earthquake swarm, while the eastern section exhibits lower deficit and slip rates, potentially due to a multilayer nappe structure. In the Maidan fault zone, the western section has a lower slip deficit and slip rate, suggesting weak crustal strength and limited stress accumulation, whereas the eastern section has higher values, correlating with the Wushi earthquake and subsequent energy release.
Finally, based on the calculated strain field and the global focal mechanism earthquake catalog(1976—2021), we applied the SHIFT_GSRM2f model developed by Bird et al. to predict shallow earthquakes. The highest predicted seismic hazard values are concentrated in the southwestern Tianshan, particularly in the Pamir region, which has a higher risk of strong earthquakes than the Keping thrust belt. However, the Keping thrust belt remains one of the most seismically hazardous areas in the Tianshan region, underscoring the continued seismic risk in southwestern Tianshan.

Key words: GNSS, Keping thrust belt, strain rate, slip rate, shallow seismicity forecast

摘要：

文中收集整理了1999—2023年柯坪冲断带及其周边地区的GNSS融合速度场结果, 利用最小二乘配置方法进行应变场计算, 结果显示柯坪冲断带位于应变高值区的边缘, 冲断带两侧的运动特征有所不同, 西侧的挤压应变高于东侧, 西侧的面应变率平均值为-3.3×10^-8/a, 东侧的面应变率平均值为-1.2×10^-8/a。综合InSAR形变结果与GNSS速度场结果, 利用三维块体模型方法计算柯坪冲断带主要断层滑动亏损速率分布及滑动速率, 结果显示, 柯坪断裂带西段的挤压速率较大, 为(3.1±0.3)mm/a, 东段的速率较小, 为(0.3±0.2)mm/a; 迈丹断裂西段的挤压速率为(2.7±0.5)mm/a, 东段的挤压速率为(3.7±0.4)mm/a。柯坪断裂带东段的滑动亏损速率较低, 可能与东侧多层推覆体构造有关。迈丹断裂带东段的亏损速率较高, 同时发生了乌什地震, 导致能量进一步释放。文中同时基于SHIFT假设和算法预测了浅源地震的发生概率, 认为柯坪地块仍处于预测概率值较高的区域, 西南天山仍是强震危险区域。

关键词: GNSS, 柯坪冲断带, 应变率, 滑动速率, 浅源地震预测