Abstract: The continuous collision between Tian Shan and Tarim Basin causes not only the uplift of mountains, but also the earthquakes across the entire Tian Shan, particularly in the transient zone from mountains to the adjacent basins, where the critical infrastructures and residents are seriously under threat from these earthquake hazards. On 19^th January, 2020, an earthquake occurred in the Kalpintage fold thrust belt in the southwest Tian Shan foreland. We call this event the 2020 M_W6.0 Kalpintage earthquake, which is the first moderate earthquake captured by modern geodetic measurement techniques. This event therefore provides a rare opportunity to look into the local tectonics and seismic risk in southwest Tian Shan. In this study, we obtained the coseismic deformation of 2020 M_W6.0 Kalpintage earthquake from Sentinel-1A SAR and strong motion data, and then inverted its kinematic slip model. We derived the InSAR interferograms from both ascending and descending tracks. Both of them present similar deformation patterns, two deformation peaks over the Kalpintage anticline. That means: 1)The surface deformation is dominated by vertical displacement, and 2)the coseismic rupture plane is highly suspected to be the shallowly dipping decollement at the base of the sediment cover. We got the 3-D displacements of 6 strong motion stations by double integrating the strong motion acceleration signals. The result shows tiny displacement on the strong motion stations, except for the Xikeer station, which locates at the front of the Kalpintage anticline, where the InSAR interferograms are seriously incoherent. Two slip models can equally fit to the ascending and descending InSAR interferograms: One is a strike slip model with strike of N-S, the other is a thrust model with strike of E-W. This ambiguity in the slip models for the M_W6.0 Kalpintage earthquake is caused by 1)the extremely small dip angles of the causative fault, 2)the inherent shortcomings of the InSAR measurements i.e. the 1-D measurements along the line of sight, the polar orbiting direction of the SAR satellite, and 3)the serious atmospheric delay due to contrasting topography in southwest Tian Shan. We did not distinguish the two ambiguous models with InSAR data due to the weak constraints of InSAR for this event. However, the two quite different slip models show the same spatial dimension and position beneath the Kalpintage anticline, also the same seismic slip vector moving toward the Tarim Basin. We then presumed the two slip models refer to the same fault plane, the weak decollement at the base of the sediment cover, and its rupture released the compressive strain in this fold and thrust belt in the southwest Tian Shan front. The confusing problem is neither the strike slip model nor the thrust model can explain the displacement derived from strong motion. The simple error estimates show small uncertainty in the strong-motion-derived displacement, but we cannot really know the real errors without the comparison to the collocated continuous GNSS observation. Because of the discrepancy between the strong motion displacement and InSAR-derived slip model, we speculate the inelastic deformation occurred in front of the Kalpintage anticline where thick weak sediments exist. We think this earthquake ruptured the decollements in the lower sediments bounded by the adjacent anticlines, which are uplifted in this event. The M_W6.0 Kalpintage earthquake balanced the stress accommodated during the convergence of the Tian Shan and Tarim Basin. We managed to explain all of the ruptures in the southwest Tian Shan by combining the regional tectonic, geophysical data and the available earthquake catalogues with good quality and then estimated the earthquake hazards. The earthquakes, including 1902 M_W7.7 karshigar, 1996 M_W6.3 Jiashi, 1997—2003 Jiashi sequence and 2020 M_W6.0 Kalpintage earthquake, can be explained in one frame, the underthrusting of the Tarim Basin toward the southwest Tian Shan. Our calculation suggests that a M_W7.0+ event could be generated around Kalpintage anticline belt if without barriers on the decollements.

Key words: 2020 M_W6.0 Kalpintage earthquake, InSAR, strong motion, earthquake risk

摘要： 塔里木盆地基底与天山造山带之间的持续挤压会聚, 不仅造就了天山不断隆升的山体, 也导致了遍布整个造山带的破坏性地震, 而位于其山体两侧盆山交接区域的破坏性地震更是严重威胁着沿线居民和基础设施的安全。 文中以2020年M_W6.0柯坪塔格地震为切入点, 获取了该地震的InSAR同震形变场, 并基于强震台加速度记录经双重积分得到了位移矢量, 以此为约束反演了本次地震发震断层的运动学参数; 此外, 综合该区域历史地震和构造特征的最新研究成果, 讨论了本次地震与历史地震之间的关系, 进而评估了柯坪塔格褶皱带周边的地震危险性。 研究表明, 2020年柯坪塔格M_W6.0地震的破裂范围受限于褶皱构造, 其产生的地表形变促进了柯坪塔格前缘褶皱的抬升, 由此推测该地震导致了褶皱带下伏沉积层底部滑脱面的位错, 是褶皱带内SN向挤压应力集中释放的表现。 包括本次地震在内的区域历史地震(1902年M_W7.7伽师地震、 1996年M_W6.3伽师地震和1997—2003年伽师地震群)均可以解释为塔里木盆地基底向西南天山下插过程中地壳对不同部位应力积累特征的响应。 经简单计算所得的结果表明, 在不考虑障碍体的情况下, 柯坪塔格周边可发生M_W7.0以上地震。

关键词: 2020年柯坪塔格地震, InSAR, 强震动, 地震危险性