Abstract: The Yishu fault zone is one of the branch faults of the Tanlu fault zone in its central part. Moderate and strong earthquakes occurred in the Yishu fault zone repeatedly. Due to its complex structure, the Yishu fault zone attracts much attention from earthquake researches. The Anqiu and Juxian electromagnetic stations in Shandong Province locate near the Anqiu-Juxian Fault and Changyi-Dadian Fault, which are branches of the Yishu fault zone, respectively. Geoelectric field and geomagnetic field observation were carried out in these two stations. The Wudi electromagnetic station is in the west of Tanlu fault zone in the Jidong-Bohai block and 230km from Anqiu electromagnetic station. This paper firstly describes the crustal structure near the electromagnetic stations by using magnetotelluric(MT)method. By processing the data carefully, we obtain the MT data in good quality near the stations. The MT data of each electromagnetic station and its nearby area suggests that the electrical structure and geological structure of the station are comparable. This paper applied 1-D and 2-D inversion for MT data and obtained the crustal electrical structure model beneath the Anqiu and Juxian seismic station. The shallow electrical structure from the MT method was compared with the results of symmetrical quadrupole electrical sounding. The model suggests that the electrical structure beneath the Anqiu and Juxian electromagnetic stations is complex and shows the feature of block boundary. The Wudi electromagnetic station is located inside a basin, the crustal structure shows layered feature typical for the stable blocks. Beneath the Anqiu electromagnetic station, there is a 1km-thick relative low resistivity layer in the shallow crust and a high resistivity body beneath it with a depth of 13km. There is a high resistivity structure in the crust beneath the Juxian electromagnetic station. The crustal structures are divided into two different parts by Anqiu-Juxian Fault and Changyi-Dadian Fault, respectively. More conductive layers appear to the west of the two faults. Plenty of fluid possibly exists within the conductive body to the west of Changyi-Dadian Fault, which plays important role in the earthquake generation. There is a relative low resistivity layer in the crust within 1~2km beneath the Wudi electromagnetic station. Beneath the relatively low resistivity layer, a relatively high resistivity layer extends to a depth of around 15km, and the resistivity value decreases with the increase of depth. The electrical resistivity model suggests the seismic activity of the Yishu fault zone around the Anqiu and Juxian electromagnetic stations should be taken into account seriously, and monitoring and research on it need to be strengthened. The results of this paper provide a certain reference value for the crustal structure research to similar stations.

Key words: Tanlu fault zone, electrial structure, extremely low frequency electromagnetic technique, magnetotelluric sounding, electromagnetic station

摘要： 郯庐断裂带中段的沂沭断裂带发生过多次中强地震，其结构复杂，是地震研究者重点关注的区域。山东安丘和莒县电磁台都具有综合地电场和地磁场观测，两台分别位于沂沭断裂带内的安丘-莒县断裂和昌邑-大店断裂上。山东无棣电磁台位于郯庐断裂带西侧，属冀东-渤海断块，距安丘台230km，但此前未曾对该台站的地壳深部电性结构进行过探测研究。文中利用大地电磁测深方法（MT）对这3个电磁台站及其附近区域的探测数据进行研究，结果表明，3个台站的地壳电性结构有明显的差别，分别与台站区域地质构造及其地震活动性存在对应关系。在每个台站及其附近的4个测点分别得到优良的视电阻率和阻抗相位等数据，经过一维和二维反演，获得了3个台站的地壳深部电性结构，并可与直流对称四级电阻率测深法得到的近地表的电性结构进行对比。无棣台及其附近具有稳定地块的成层性地壳深部结构，而安丘和莒县电磁台分别具有复杂的地壳电性结构，呈现出活动地块边界带的结构特点。安丘台和莒县台附近区域可分别以安丘-莒县断裂和昌邑-大店断裂为界，将地壳电性结构分成2部分，在断裂西侧即郯庐断裂带内部的中下地壳低阻层更发育，昌邑-大店断裂西侧的低阻体可能含丰富的流体，在区域的地震孕育发展过程中起着重要作用。

关键词: 郯庐断裂带, 电性结构, 极低频电磁探测技术, 大地电磁测深, 电磁台站