Abstract: On 31 July 1954, an M_S7.0 earthquake occurred southeast of Minqin, Gansu Province, northwestern China. Its epicenter was located at the edge of the Alxa block, subject to northeastward compression of the Tibetan plateau, resulting in active tectonics there. Because of few records and field investigations, the seismogenic fault and tectonic setting of this event remain unclear. To probe the deep structure of this region, magnetotelluric (MT) measurements have been carried out near the epicenter, and new data of 28 sites were collected. Using the methods including the remote reference, "robust" and phase tensor decomposition, these MT data were processed, followed by NLCG two-dimensional inversion of the data to reveal the deep electrical structure of the study area. Combining with previous studies, geologic interpretation of the MT survey suggests that the Minqin earthquake of 1954 may be related to the Hongyashan-Sidaoshan Fault, which is a high-angle thrust with left-slip component. It lies between the Tibetan plateau and the Alxa block, where substantial elastic strain has accumulated due to the northeastward extrusion of the plateau, leading to occurrences of several earthquakes greater than M_S5.0 in the history. Our electrical structure derived from the MT survey supports the following tectonic interpretations:The Tibetan plateau expands to the northeast in a flower-like style while the Alxa block subducts to southwest in a listric-shaped manner, which forms the northeastward growth pattern of the Tibetan plateau. The forefront of the plateau expansion is around the Hongyashan-Sidaoshan Fault, indicating that the extension of the plateau has surpassed the Hexi Corridor to the southern margin of the Alxa block. The deformation nearby the Hongyashan-Sidaoshan Fault could be linked to the northeastward propagating extrusion of the Tibetan plateau as a far-field dynamic effect of the India-Eurasia collision. The Tibetan plateau is continuing to grow northeastward, resulting in folds and thrusts in the Hexi Corridor, and even farther to the southern margin of the Alxa block.

Key words: magnetotelluric, Minqin M_S6.6 earthquake, Tibetan plateau, northeastward expansion, Hongyashan-Sidaoshan Fault

摘要： 1954年7月31日，在甘肃省民勤县东南部发生了1次7级地震，震中处于青藏高原NE向挤压阿拉善地块的前缘部位，构造活动强烈。由于此次地震的记录、地表地质调查和其他研究资料都较少，关于此次地震的发震断裂以及孕震环境还有诸多不确定。为研究这一地区的深部构造，在古浪8级地震区的大地电磁剖面北段的基础上进行了加密测量，共获得了28个测点数据。文中利用先进的处理与解释技术对这些数据做了反演，得到了研究区的深部电性结构。参考前人的研究结果，地质解释认为1954年民勤7级地震的发生可能与红崖山-四道山断裂（F₁）有关，此断裂表现为高角度逆冲为主兼具左旋走滑特性的逆冲走滑断裂带，是在青藏高原地块与阿拉善地块之间形成的巨大的地块边缘碰撞断裂带，在青藏高原NE向的扩展过程中积累了大量的应变能量，成为构造应力集中之处，导致此地区有历史记录以来发生多次5级以上地震。青藏高原地块以单侧北向"花状"向NE向扩展，阿拉善地块呈"铲状"向SW侧俯冲挤压，它们的相互作用控制了青藏高原的NE向生长，扩展最前缘位于红崖山-四道山断裂（F₁）附近，表明青藏高原向N向扩展的活动前缘可能已经延伸至河西走廊北部的阿拉善南缘区域。红崖山-四道山断裂（F₁）附近的构造变形代表着欧亚大陆碰撞的远程效应，表明青藏高原正在向更远的阿拉善地块南部继续扩展。

关键词: 大地电磁, 民勤7.0级地震, 青藏高原, NE向扩展, 红崖山-四道山断裂