DataSheet1_3D deep electrical structure and seismogenic environment in the western section of the Zhangjiakou-Bohai fault zone.docx

The Zhangjiakou-Bohai fault zone (ZBFZ) is an important NW-trending active tectonic zone in North China. The western section of ZBFZ is characterized by frequent moderate and strong earthquakes. This is a typical tectonic area for studying seismic and volcanic activities. The three-dimensional (3D) deep electrical structure of the region was revealed by using 3D electromagnetic inversion of 143 magnetotelluric stations. The results suggest that the deep electrical structure in the western part of the ZBFZ is laterally heterogeneous. To the north of the Shangyi-Chicheng Fault (F4), the upper crust has a high resistivity structure (R), corresponding to the exposed Hanuoba basalts and metamorphic rocks. To the south of F4, high and low resistivity zones alternate, which is attributed to the complex faults and basin-range structures. In the middle crust, the low-resistivity structures (C1 and C2) gradually appear in the northwest and southeast of the study area, showing the characteristics of a high-resistivity structure sandwiched between two low-resistivity structures. In the lower crust, the two low-resistivity structures (C1 and C2) gradually join each other and the whole region becomes characterized by low resistivity. The Shangyi-Chicheng Fault (F4), the northern margin fault of the Huai’an Basin (F7), and the northern margin fault of the Huaizhuo Basin (F11) are definite electrical difference zones, which are connected to a low-resistivity layer in the crust. The results indicate that the focal depth of the 1998 M6.2 Zhangbei earthquake was between 12 km and 15 km and its seismogenic fault was the Dahezhen-Hailiutu Fault (F3), which is concealed beneath the Hanuoba basalt area. It is speculated that owing to the neotectonic activities, the magma originated in the asthenosphere mantle rose and erupted on the surface near Datong, forming the Datong volcanic group. Then the magma continued to migrate and overflow along with the fractures in the NE direction, forming the Hannuoba basalts in the Zhangbei area. The existence of mantle thermal material in this area led to the migration of accompanying fluids to the middle and upper crust, resulting in fault sliding and frequent occurrence of moderate and strong earthquakes in this region.

张北-渤海断裂带（ZBFZ）是华北地区一条重要的西北走向的活跃构造带。该断裂带的西部区域以频繁发生的中强地震为特征。这一区域是研究地震和火山活动典型的构造地带。通过143个磁大地电测站的三维电磁反演，揭示了该地区的三维深部电性结构。研究结果表明，ZBFZ西部区域的深部电性结构在横向上呈现非均质性。在尚义-赤城断裂（F4）以北，地壳呈现出高电阻率结构（R），这与出露的汉诺坝玄武岩和变质岩相对应。在F4以南，高电阻率和低电阻率区域交替出现，这归因于复杂的断裂和盆山结构。在中地壳，低电阻率结构（C1和C2）逐渐出现在研究区域的西北部和东南部，显示出被两个低电阻率结构夹持的高电阻率结构特征。在下地壳，两个低电阻率结构（C1和C2）逐渐连接，整个区域呈现出低电阻率特征。尚义-赤城断裂（F4）、淮安盆地北缘断裂（F7）和怀珠盆地北缘断裂（F11）是明显的电性差异区，它们与地壳中的低电阻率层相连。研究结果表明，1998年M6.2级张北地震的震源深度在12至15公里之间，其发震断层为大石庄-海力图断裂（F3），该断层位于汉诺坝玄武岩区下方。推测由于新生代构造活动，来自地幔软流圈的岩浆上升并在大同附近地表喷发，形成了大同火山群。随后，岩浆继续沿东北方向的裂缝迁移和溢出，形成了张北地区的汉诺坝玄武岩。该区域存在的地幔热物质导致了伴随流体的迁移至中上地壳，导致断层滑动，并频繁发生中强地震。