Abstract:

Based on the GNSS velocity field, we analyzed the present-day crustal deformation characteristics of Xinjiang and its adjacent regions before the January 23, 2024, Wushi earthquake using the spherical least squares configuration method. Based on the fundamental concept of active blocks, Xinjiang and its neighboring areas were divided into 17 active blocks by integrating regional seismic geological data. The slip rates of the boundary fault zones of these active blocks were then calculated using a three-dimensional elastic block model. Based on this block delineation, the study area was further divided into 91 potential seismic hazard zones. We incorporated geodetic observations, including fault strike-slip rates and dilatational strain rates derived from the GNSS velocity field, into the conventional probabilistic forecasting of significant seismic hazards. The relationship between the Wushi 7.1 earthquake and the regional crustal deformation characteristics, as well as the prediction results of the probability of strong earthquakes, was comprehensively analyzed.
The direction and magnitude of GNSS velocity field motions in the study area under different dynamical backgrounds are distinctly characterized by zoning. GNSS stations east of longitude 76° move toward the north or northeast, while those on the west side move north or northwest. The velocity field difference is primarily evident on the north and south sides of the Tianshan Mountain. The velocity change of GNSS sites from the Tibetan plateau to the Tarim Basin is minor, but it significantly decreases after crossing the Tianshan Mountain, indicating that the Tianshan Mountain tectonics absorb most of the remote effects of the Indo-Eurasian plate collision. The principal strains obtained from the least-squares configuration results reveal that the extrusion characteristics are most prominent near the western section of the Southern Tianshan Mountains and the Altyn Tagh fault zone. In the western section of the Southern Tianshan Mountains, the direction of the principal compressive strains is perpendicular to the tectonic direction in the region. This suggests that the region is primarily influenced by a force perpendicular to the tectonic direction, resulting in the predominant retrograde movement of major fault zones in the area. Moreover, in addition to the western section of the Southern Tianshan Mountains, the region with a greater main compressive strain is the Altyn Tagh fault zone. The direction of the main compressive strain intersects obliquely with the Altyn Tagh fault zone, suggesting that the force background is linked to the left-lateral reverse slip movement of the Altyn Tagh fault zone.
The results derived from the inversion of the three-dimensional elastic dislocation model reveal that the motion features of the main active faults within the study area indicate a predominance of dextral slip along the northwest-trending fault zones in the Tianshan region, while sinistral slip motion is primarily observed along the northeast-trending or northeastern fault zones. Apart from the Altyn Tagh fault zone, the strike slip rate of the dextral-slip fault zones is notably higher than that of the sinistral-slip fault zones. The fault zones at the northern edge of the Junggar Basin and the major faults in the Tianshan region are primarily characterized by extrusion movements. The extrusion rate of the fracture zones in the South Tianshan Mountain is higher than that in the North Tianshan Mountain. Specifically, the west section of the Keping fault zone and the west section of the Nalati fault zone exhibit the most prominent extrusion movements.
The study area has been divided into 91 potential seismic hazard zones based on block delineation and the findings from previous research. The slip rates and regional surface strains of the major faults obtained from the inversion are utilized in classical probabilistic predictions to derive quantitative results regarding the strong earthquake hazard in the study area over the next 50 years. The results indicate that the strong earthquake hazard is primarily concentrated in the western section of the Southern Tianshan region. This includes areas such as the north-east or nearly east-west-trending Maidan fault, the Nalati fault, and the Usun Ridge Fault, as well as the north-west-trending Talas-Fergana fault zone and the north-west section of the Kyzyltau fault zone. Furthermore, the probability of strong earthquakes is elevated in the northern Luntai fault compared to the surrounding faults. In the northern Tianshan region, areas with relatively high probabilities of strong earthquakes include the Fukang fault and the western section of the Bogda fault.
The seismic mechanism solution reveals that the M7.1 Wushi earthquake was a thrust earthquake, aligning with the characteristics of the Maidan fault zone. In this zone, the seismogenic fault is primarily influenced by extrusion motion. The Wushi earthquake occurred in the Maidan fault zone, situated at the border of the high shear strain rate zone and the high probability hazard zone. This occurrence validates the effectiveness and accuracy of the probabilistic prediction method.

Key words: Wushi earthquake, crustal deformation, strain rate, block model, strike-slip rate, probability prediction

摘要：

文中基于GNSS速度场结果利用球面最小二乘配置方法计算获取2024年1月23日乌什 M_S7.1 地震前新疆地区现今的地壳变形特征。基于活动块体基本概念, 结合区域地震地质数据将新疆及其邻区划分为17个活动块体, 利用三维弹性块体模型计算了活动块体及其边界断裂带的滑动速率。基于块体划分结果, 将研究区划分为91个潜在地震危险区。将基于GNSS速度场反演得到的断层滑动速率、 面应变率等大地测量结果应用到经典强震危险性概率预测中, 给出各潜在危险区的强震危险性概率预测结果, 并综合分析了乌什7.1级地震与区域地壳变形特征和强震概率预测结果之间的关系。结果表明, 新疆地区的速度场、 应变率场和主要边界断裂带的滑动速率均具有明显的分区特征。速度场方向变化与动力背景有关, 大小变化与天山构造带的吸收作用有关。南天山西段主压应变特征最为明显, 其次是阿尔金断裂带。各块体边界断裂带中NW走向的断裂带以右旋走滑运动为主, NE或近EW向的断裂带以左旋走滑运动为主, 整个天山地区以挤压变形为主。基于乌什地震震前数据获取的高概率危险区段主要集中在南天山西段, 包括NE走向的迈丹断裂带、 那拉提断裂带和乌孙山脊断裂带, NW走向的塔拉斯-费尔干纳断裂带和克孜勒陶断裂带北西段等。北天山强震危险概率相对较高的区域包括阜康断裂带、 博格达断裂带西段等。乌什7.1级地震发生在高发震概率迈丹断裂带上, 验证了方法的有效性。

关键词: 乌什地震, 地壳变形, 应变率, 块体模型, 滑动速率, 概率预测