Abstract:

On January 8, 2022, an earthquake of M_W6.7 occurred in Menyuan County, Qinghai Province. The epicenter of the earthquake is located in the middle eastern section of the Qilian Mountains seismic belt on the northeast edge of the Qinghai-Tibet Plateau. Under the northward push by the Qinghai-Tibet Plateau plate, and the push and subduction in the northeast direction of the Qilian Mountains, and also blocked by the Alxa block and affected by the push in the southwest direction of the Longshou Mountains uplift area at its front edge, a ramp structural pattern of compressive depressions was formed in the Hexi Corridor basins area. As a result, most of the active faults in this area are mainly NW-trending, and the active features are mostly characterized by compressional thrusting and strike slip. This paper reconstructs the coseismic deformation field of Menyuan earthquake through the European Space Agency Sentinel-1A C-band radar satellite data and D-InSAR technology, determines the geometric characteristics of the seismogenic fault through the inversion method of optimum fault slip distribution, and determines the seismogenic fault of this earthquake. The results show that the deformation range of the radar line of sight is -0.42~0.7m for the ascending track deformation field and -0.63~0.72m for the descending track deformation field, and the maximum deformation locates in the Lenglongling section. The data of ascending and descending tracks show that there are two obvious deformation regions with a butterfly-like stripe pattern. The sign of LOS deformation variable observed in InSAR deformation field of ascending and descending orbits in the same area is opposite. Combined with the flight direction of ascending and descending satellites, it is determined that the motion of seismogenic fault is mainly left-lateral strike slip. Among them, the Lenglongling Fault and Tolaishan Fault pass through the fracture surface revealed by InSAR deformation field, which means that the above fault is highly likely to be the seismogenic fault of the Menyuan earthquake in 2022. At the same time, the SE-trending Lenglongling Fault on the east side passes through the fracture surface, with a surface fracture length of about 20km. The EW-trending Tolaishan Fault on the west side also passes through the fracture surface, with a fracture length of about 5km. And then, according to the field geological survey results of this earthquake, taking the InSAR coseismic deformation field data as constraints and based on Okada elastic dislocation model, the geometric structure of the seismogenic fault and the fine slip distribution characteristics of the fracture surface are determined. The inversion results reveal that there are two slip regions, of which the slip is mainly concentrated in the Lenglongling fault section, with a maximum left-lateral slip of 3.66m and a maximum slip depth of 5km. There is also a maximum sinistral slip of 1.95m occurring at a depth of 5km in the Tolaishan Fault. It is inferred that the seismogenic fault is the western section of Lenglongling Fault which also ruptured the Tolaishan Fault on its west.

On this basis, Coulomb33 software is used to calculate the static Coulomb stress changes generated by the Menyuan earthquake at different depths(5km, 10km, 15km and 20km). The Coulomb stress change image within 300km of the epicenter shows a typical four-quadrant distribution characteristic. There are four fan-shaped stress increase and decrease areas at 5km underground. The area with the largest increase in stress is near the Beiyuan Fault of Tuole Mountain in the west of the epicenter of Menyuan earthquake. The increase of stress is over 0.03MPa, greater than the trigger threshold of 0.01MPa. The stress increase coverage area in the south of the epicenter further expanded, with a stress increase of more than 0.03MPa, inducing many aftershocks distributed linearly in the NWW direction along the epicenter. According to the overall analysis, most of the subsequent earthquakes in Menyuan occur at a depth of 10~12km, which is in good agreement with the stress increase area at the corresponding depth. At the same time, for the NW-SE area and NE-SW end of the rupture in the epicenter, the area with ΔCFS≥0.01MPa is worthy of attention for the subsequent risk.

Finally, based on the GPS velocity field relative to the Ordos block, it is analyzed that the Lenglongling area moves in the NE direction relative to the Ordos block as a whole, the GPS velocity vector north of the Lenglongling Fault decreases, and the movement direction turns to NNW. Using GPS velocity field to calculate the principal strain rate, shear strain rate, surface strain rate and principal compressive stress in Lenglongling area, it is shown that there is a significant high value area of surface strain in Lenglongling area. The principal strain rate is NE compression, and the peak value of shear strain rate is located on the north side of Lenglongling Fault and the east section of Minle-Damaying Fault. Its strain accumulation indicates that the area is still in a high stress state, and the seismic activity may continue to be strong in the future. The regional surface strain rates show obvious compression characteristics, and the principal strain rates show NE-SW compression and NW-SE extension. Overall, the source area of the Menyuan earthquake is still under the push in the NNE direction of the eastern Himalaya syntaxis of the Indian plate. It can also be seen that the Menyuan earthquake occurred in the high value area of the maximum shear strain rate and the compression area of the surface strain. The occurrence of the Menyuan earthquake in the Lenglongling area of the North Qilian Mountains on the northeast edge of the Qinghai-Tibet Plateau and its current high stress accumulation indicate that the Lenglongling Fault may still be active today.

On this basis, the seismogenic structural characteristics and seismogenic relationships of the two Menyuan earthquakes in 2016 and 2022 are further discussed. The 2016 Menyuan earthquake is located on the extension line of the Minle-Damaying Fault. The seismogenic fault is a SW-trending thrust fault. The fault extends in NW-SE direction on the surface along the front of the mountain, and its deep part may converge to the detachment layer at the bottom of the Qilian Mountains together with the Lenglongling Fault. The fault has the potential to generate destructive earthquakes. The 2016 M_W5.9 Menyuan earthquake and the 2022 M_W6.7 Menyuan earthquake have different seismogenic mechanisms, but the seismogenic faults all belong to the North Qilian Mountains active fault zone, most of which control the boundary of the Neogene basins. Both earthquakes are the local adjustment of stress accumulation in the region as a whole, and the expression of the northeastward pushing of the Qinghai Tibet Plateau. Some scholars believe that Lenglongling fault zone, Jinqianghe Fault, Maomaoshan fault zone and Laohushan Fault jointly constitute the “Tianzhu earthquake gap”. The occurrence of three Menyuan earthquakes in 1986, 2016 and 2022 has drawn continuous attention to the fault activity and seismic risk of this area.

Key words: Menyuan earthquake in 2022, Sentinel-1 InSAR, coseismic inversion, seismogenic fault, Qinghai-Tibet Plateau, static Coulomb stress change

摘要：

2022年1月8日, 青海省门源县发生 M_W6.7 地震。文中运用Sentinel-1A数据, 采用 InSAR 技术获取震区的LOS向形变场, 其中最大形变量分别为7.0cm和7.2cm, 结合升、 降轨卫星的飞行方向, 判定发震断层的运动性质以左旋走滑为主, 其中最大形变量位于冷龙岭破裂段。此外, 以InSAR形变场数据为约束, 基于Okada弹性位错模型, 厘定了发震断层的几何结构及破裂面的精细滑动分布特征, 反演结果揭示出2个断层破裂面。 冷龙岭破裂段是滑动主要集中的区域, 最大左旋滑动量为3.66m, 最大滑动深度为5km; 而托莱山断裂处存在1.95m的左旋滑动量, 位于5km深度处。 判定发震断层为冷龙岭断裂西段, 地震同时使托莱山断裂发生破裂。在此基础上, 计算了库仑应力变化, 结果显示震中300km区域内的库仑应力变化图像呈现走滑型地震特有的四象限分布特征。同时, 震中破裂的NW-SE区域和NE-SW端的ΔCFS≥0.01MPa, 这些区域后续的地震危险性值得关注。最后, 相对于鄂尔多斯块体的GPS速度场显示冷龙岭地区存在一个显著的面应变高值区, 未来该区的地震活动性可能持续较强。 此外, 文中还讨论了2016年和2022年2次门源地震的发震构造特征及发震关系, 2次地震整体都是该地区应力积累的一次局部调整, 它们都是青藏高原向NE推挤运动的表现。

关键词: 2022年门源地震, 哨兵InSAR, 同震反演, 发震断层, 青藏高原, 静态库仑应力变化