2022年1月8日青海门源MS 6.9地震地表破裂考察的初步结果及对冷龙岭断裂活动行为和区域强震危险性的启示

2022年1月8日青海门源M_S 6.9地震发生在青藏高原东北缘的祁连山断块内部，仪器震中位于海原活动断裂系西段的冷龙岭断裂带上，是该断裂系自1920年海原8.5级大地震后再次发生M>6.5的强震。考察结果的初步总结表明，此次门源地震产生了呈左阶斜列分布、总长度近23 km的南北两条破裂，在两者之间存在长约3.2 km、宽近2 km的地表破裂空区。南支破裂(F₁)出现在托来山断裂的东段，走向91°，长约2.4 km，以兼具向南逆冲的左旋走滑变形为主，最大走滑位移近0.4 m。北支主破裂(F₂)出现在冷龙岭断裂的西段，总长度近20 km，以左旋走滑变形为主，呈整体微凸向北东的弧形展布，包含了走向分别为102°、109°和118°的西、中、东三段，最大走滑位移出现在中段，为3.0±0.2 m。此外，在北支主破裂中—东段的北侧新发现一条累计长度约7.6 km、以右旋正断为主的北支次级破裂(F₃)，累计最大走滑量约0.8 m，最大正断位移约1.5 m。综合分析认为，整个同震破裂以左旋走滑变形为主，具有双侧破裂特点，宏观震中位于北支主破裂的中段，其地表走滑位移很大可能与震源破裂深度浅有关，其中的右旋正断次级破裂可能是南侧主动盘向东运移过程中拖曳北侧块体发生差异运动所引起的特殊变形现象。印度与欧亚板块近南北向强烈碰撞挤压导致南祁连断块沿海原左旋走滑断裂系向东挤出，从而引发该断裂系中的托来山断裂与冷龙岭断裂同时发生破裂，成为导致此次强震的主要动力机制。在此大陆动力学背景下，以海原左旋走滑断裂系为主边界的祁连山断块及其周边的未来强震危险性需得到进一步重视。 

Surface rupture investigation of the 2022 Menyuan MS 6.9 Earthquake, Qinghai, China:Implications for the fault behavior of the Lenglongling fault and regional intense earthquake risk

The M_S 6.9 Menyuan earthquake on January 8, 2022 occurred in the Qilian Mountain fault block in the northeastern margin of the Tibetan Plateau. The instrumental epicenter is located in the Lenglongling fault zone in the west section of the Haiyuan active fault system. It is another strong earthquake with M>6.5 that occurred in the fault system after the Haiyuan M 8.5 earthquake in 1920. The preliminary conclusion of the investigation results shows that the Menyuan earthquake generated two main fracture zones in the south and north separately, which are distributed in left-step oblique arrangement with a total length of nearly 23 km, and there is a surface rupture cavity with a length of about 3.2 km and a width of nearly 2 km between them. The south branch rupture (F₁) appears in the east section of the Tuolaishan fault, striking 91°, with a length of about 2.4 km. It is mainly characterized by southward thrust and sinistral strike slip, with a maximum horizontal displacement of nearly 0.4 m. The north branch main rupture (F₂) appears in the west section of the Lenglongling fault, with a total length of nearly 20 km. It is mainly sinistral strike-slip deformation, and presents an overall slightly convex arc distribution to the northeast, including the west, middle and east sections of 102°, 109° and 118°, respectively. The maximum strike-slip displacement occurs in the middle section, which is 3.0±0.2 m. In addition, a new north branch secondary fault (F₃) with a cumulative length of about 7.6 km, dominated by normal-dextral fault, is discovered on the north side of the middle to east section of the main fault of the north branch, with a cumulative maximum horizontal displacement of about 0.8 m and a maximum normal vertical displacement of about 1.5 m. Comprehensively, the whole coseismic rupture is mainly sinistral strike-slip and has the characteristics of bilateral rupture, whereas the macroscopic epicenter is located in the north branch of midway through the main fracture zone, thus the strike-slip displacement of the surface may be associated with the shallow rupture of the source. The normal-dextral rupture is likely to be caused by the differential movement of the north block dragged by the active south plate. The south Qilian block was extruded eastward along the Haiyuan sinistral strike-slip fault system due to the strong collision and compression between the Indian and Eurasian plates, which led to the simultaneous rupture of the Tuolaishan fault and the Lenglongling fault. This is the main dynamic mechanism leading to the strong earthquake. Under the background of continental dynamics, further attention should be paid to the future strong earthquake risk of the Qilian Mountain fault block and its surrounding area with the Haiyuan sinistral strike-slip fault system as the main boundary.