Abstract: The Tancheng-Lujiang Fault is an important tectonic boundary in eastern China. The southern part of the Tancheng-Lujiang Fault is located south of Baohai Bay, which is an area with a dense population and frequent economic activities. It is worth conducting an in-depth study on the southern section of the fault, especially in the aspect of geophysical exploration and seismicity analysis. Electrical structure detection is an important way to interpret the structural activity of the fault. It can also analyze and explore the influence of the fault on the physical properties of both sides of the fault based on the geoelectrical observation data. In the study area, there are densely distributed stations of geoelectrical observation, including 27 fixed stations distributed along the fault zone from the southern Baohai Bay to Nanjing, Jinagsu Province. The continuous observations and recording of these stations provide a favorable condition for studying the tectonic activity of Tancheng-Lujiang Fault.
In the long-term observation of geoelectric observation network, the geoelectric field measurements of long- and short-spacing measuring tracks in the same direction at the same station vary significantly because of the effect of long-term stability of the observation system and the environment near the electrodes. Also, the data curve changes complicatedly and seems to be in a mess. However, there are three basic facts of observation existing in the geoelectric field change: 1)The variation amplitude of the geoelectric field changes observed on the long- and short-spacing measuring tracks in the same direction at the same station(including tidal response changes and the rapid change events such as short periods or pulses)is the same or very close; 2)The E_x and E_y components at the same station always show the same variation in the same time period, or the opposite, which is related to the anisotropy of the medium under the station; 3)The rapid changes of the minute values of the geoelectric field observed at different stations are synchronous in a wide spatial area. In this study, in order to take full advantage of these basic facts, we only use the amplitude variation of geoelectric field with time.
Based on the data of 27 geoelectric field observation stations in the study area, we used the current density vector and streamline to characterize telluric current with its divergence and vorticity calculated in the southern Tancheng-Lujiang Fault in this paper. The results show that: 1)the telluric current shows the phenomenon of opposite directional differentiation in the southern part of the fault zone, the direction of the current vector is NE on the east side, while the direction is NW to SW on the west side; (2)The divergence and vorticity of telluric current also show the differentiation phenomenon along the fault, the positive/negative maximum of vorticity and divergence mainly occurs near the fault zone and the direction of alternating positive and negative gradient(or negative gradient)of vorticity or divergence is consistent with the strike of the fault zone. By analyzing the current superposition simulation results and comparing them with previous studies, an interpretation model of the above phenomenon is established in this paper. The results agree with previous studies on the electrical structure of this region. Besides, the results that telluric current differentiates along the fault zone may improve our understanding of the process of deep electrical and material migration.

Key words: the southern Tancheng-Lujiang Fault, telluric current, direction differentiation, current superposition simulation

摘要： 文中以渤海湾以南的郯庐断裂带南段为研究对象, 应用华北地区27个地电场台站的观测数据, 计算大地电流密度矢量和流线场, 并获取流线场的散度和涡度。 结果显示: 郯庐断裂带南段对表层大地电流的流向有分异作用, 在郯庐断裂带两侧的表层大地电流矢量方向相反, 东侧流向为NW向, 西侧流向为SW或W向; 断裂带对流线场的散度和涡度也存在分异作用, 散度和涡度的正(负)最大值主要出现在断裂带附近的浅层高导区域。 文中通过模拟断裂带两侧的磁感应电流, 并与前人的研究进行对比, 建立了断裂带分异作用的解释模型。 该模型显示电流方向的差异与郯庐断裂带南段深部的电性结构相关, 电性差异是近地表大地电流方向分异的主要原因。 研究断裂带对表层电流的分异现象对于理解深部电性差异具有参考意义。

关键词: 郯庐断裂带南段, 大地电流, 分异作用, 数值模拟