Abstract:

The Haiyuan-Liupanshan tectonic belt is one of the most significant tectonic deformation areas in the northeastern Qinghai-Tibetan plateau with frequent strong earthquakes. It is an important opportunity to study the northeast extension of the Qinghai-Tibetan plateau and an ideal place to study the earthquake breeding process.

The published GPS observations show that the southwest side of the Haiyuan fault may still be undergoing deformation caused by the crustal viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake. And the publicly published leveling data results show local vertical deformation of the crust in the area west of the Liupanshan fault is significant. According to the seismic geological data, there exist historical earthquake rupture gaps in the middle and south sections of the Liupanshan fault and the southeast section of the Xiangshan-Tianjingshan fault in the Haiyuan-Liupanshan structural area, which have the background of strong earthquakes above M7.0. In view of the low spatial resolution of GPS and leveling observations, we need to use high-resolution crustal deformation fields to further study the crustal deformation characteristics of the above regions. Therefore, we further discuss the above issues in combination with InSAR observations.

The Sentinel-1A/B SAR data of two orbits covering the Haiyuan-Liupanshan fault from 2014 to 2020 were processed to obtain the current crustal deformation field in the line-of-sight direction. Furthermore, the high-density regional crustal deformation field was obtained by integrating InSAR and published GPS observations of the horizontal crustal movement velocity field on a time scale of 20 years. By comparing the observations of GPS, leveling and InSAR and high-resolution three-dimensional deformation integrated GPS-InSAR field, the characteristics of crustal deformation and strain field in the region are analyzed and discussed. The main conclusions are as follows:

(1)GPS and InSAR observations show that the post-seismic viscoelastic relaxation effect of the 1920 Haiyuan M8.5 earthquake may still be pronounced on the south side of the Haiyuan fault, but this conclusion is still speculative and needs to be confirmed by further observations;

(2)The high-resolution horizontal deformation field from GPS-InSAR shows that the decrease of the sinistral slip rate of the Haiyuan fault along the fault strike mainly occurs in the Middle East section. In contrast, the decrease of the middle and west sections is not significant, which may be related to the transformation of the left-lateral strike-slip to thrust nappe structure between the Haiyuan fault and the Liupanshan fault.

(3)GPS vertical and leveling observations both show that the vertical crustal deformation characteristics in the middle and south sections of the Liupanshan fault are similar to the vertical deformation of the Longmenshan fault before the Wenchuan earthquake. Considering the similar structural characteristics of the Liupanshan fault and the Longmenshan fault, and combining with the seismic and geological data, we believe that the Liupanshan fault may be in the relatively late stage of the earthquake breeding process. It can also be recognized by the high-resolution horizontal deformation and strain field derived from GPS-InSAR data. According to the fault motion parameters obtained in our study and the existing seismic and geological data, it is estimated that the maximum moment magnitude of an earthquake in the middle-south section of Liupanshan Mountain is approximately 7.5.

(4)The areas with rapid maximum strain accumulation in the study region are mainly concentrated in the vicinity of the Haiyuan fault and the left lateral shear zone between the Haiyuan fault and the Xiangshan-Tianjingshan fault. The dilatation strain rate west of the Liupanshan fault shows prominent compressive deformation characteristics corresponding to the nappe deformation in the Liupanshan tectonic area. The strain rate field in the southeast section of the Xiangshan-Tianjingshan fault is smaller than that of the surrounding area. There is a strain mismatch phenomenon, which may be related to the preparation for strong earthquakes. From the perspective of rotational deformation, the study area presents multiple deformation units, among which counterclockwise rotation corresponds to left-lateral strike-slip deformation(the left-lateral shear belt from the Haiyuan fault to the Xiangshan-Tianjingshan fault). In contrast, clockwise rotation corresponds to right-lateral strike-slip deformation(the right-lateral shear belt in the western margin of Ordos and Longxi block).

Key words: the Haiyuan-Liupanshan Fault, the Great Haiyuan earthquake of 1920, GPS, InSAR, leveling

摘要：

海原-六盘山构造区为青藏高原东北部构造变形最为显著的区域之一, 历史强震活动频繁, 是研究青藏高原NE向扩展的重要窗口和地震孕育过程的理想场所。文中处理了跨海原-六盘山断裂2014-2020年期间2个轨道的时序Sentinel-1A/B SAR数据, 获得了该区域InSAR视线向现今的地壳形变场。融合公开发表的近十多年时间尺度的水平GPS地壳运动速度场, 获得了研究区高密度地壳水平形变场。对比GPS、水准和InSAR观测结果, 以及GPS-InSAR融合的高密度水平形变场, 分析讨论了该区域的地壳形变、应变场特征及其与构造之间的对应关系。主要结论如下: 1)GPS和InSAR观测表明, 1920年海原8.5级大地震的震后黏弹性松弛效应在海原断裂南侧至今仍较为明显; 2)GPS-InSAR高分辨率水平形变场表明, 狭义海原断裂左旋滑动速率的递减主要发生在中东段, 而中西段递减并不显著, 可能与海原断裂向六盘山断裂之间由左旋走滑向逆冲推覆构造转换有关; 3)六盘山断裂中-南段的地壳垂直形变和水平形变场特征均显示, 该段断裂可能处于强震孕育的中晚期, 根据反演得到的断层运动参数和地震地质资料, 估算六盘山断裂中-南段发生强震的最大矩震级约达7.5级; 4)研究区应变积累较快的区域主要集中在海原断裂附近和海原断裂-香山-天景山断裂之间的左旋剪切区, 香山-天景山断裂东南段的应变率场和周围相比明显偏小, 存在应变不匹配现象, 可能与强震孕育有关。

关键词: 海原-六盘山断裂, 1920年海原大地震, GPS, InSAR, 水准