Abstract:

In order to study the deep electrical structure and its formation mechanism of different tectonic units in the central and southern part of the North China Depression, especially the southern North China Depression with NW-NNW trending tectonic lines and the northern North China Depression with NE-NNE trending tectonic lines, as well as the deep tectonic background of three destructive earthquakes in the study area, a 110km long magnetotelluric(MT)sounding profile across the main structural units in the study area was deployed to study the deep fine electrical structure in the central and southern part of the North China Depression by using high-density and broadband MT method with the support of the active fault exploration project in Kaifeng City. The profile is NS-trending as a whole, starting from Tongxu County of Kaifeng City in the south, passing through Xiangfu District of Kaifeng City, Fengqiu County and Changyuan County of Xinxiang City, and terminating at Banpodian Township of Huaxian County of Anyang City in the north, with a total length of 110km and an average point distance of about 3km. The observation points in the survey area of Kaifeng City are dense, with a point distance of 2km. From south to north, the whole profile crosses two first-order tectonic units of the southern North China Block and the northern North China Block and four second-order tectonic units of the Taikang Uplift, the Kaifeng Depression, the Dongpu Depression and the Neihuang Uplift. In MT data processing, in addition to the remote reference and robust techniques, the multi-point and multi-frequency tensor decomposition was employed to determine the regional electric strike, and the NLCG 2D inversion was performed on TE and TM data. And finally, the deep electrical structure is obtained.

The result shows that, with Xinxiang-Shangqiu Fault as the boundary, the deep electrical structure on its north is relatively simple than that on the south. The electrical structure of Neihuang Uplift and Dongpu Depression in the northern North China Depression is relatively simple, and its resistivity structure is characterized by vertical segmentation and divided into low resistivity and high resistivity zones corresponding to the crust of the area consisting of sedimentary cover and crystalline basement of hard block with good basement integrity. At the same time, the high resistivity zone is very thick, which could represent the unified crystallization basement in the North China Block region. The deep electrical structure of the tectonic units in the southern North China Depression on the south of Xinxiang-Shangqiu Fault is relatively complex, showing a three-layer structure of low-high-low resistivity in the vertical direction and alternating high and low resistivity in the horizontal direction. For example, the resistivity of the crust below the Taikang Uplift shows a low-high-low three-layer structure, that is, a low resistivity zone above the depth of 2km, a high resistivity zone at the depth between 2km and 15km, and a low resistivity zone below 15km. This may be related to the mutual subduction and collision between the Yangtze plate and the southern margin of the North China plate, while the northern North China Depression is less affected by the Yangtze plate and Qinling-Dabie orogeny due to the control of the boundary Xinxiang-Shangqiu Fault.

The two destructive earthquakes of 1342 and 1918 in Tongxu, Henan Province are located in the intersection area of high and low resistivity zones beneath the Taikang Uplift, and the Fengqiu earthquake of 1737 in Henan Province is located near the gradient zone of high and low resistivity in the crust.

Key words: North China Depression, electrical structure, Xinxiang-Shangqiu Fault, cause of earthquake

摘要：

为研究华北坳陷中南部河南地区的深部电性结构和该区3次破坏性地震的深部孕震背景, 采用宽频带大地电磁测深法对该区深部的电性结构进行探测。使用NLCG二维反演方法对TE、TM模式进行联合反演, 最终获得研究区的深部电性结构。结果显示, 以新乡-商丘断裂为界, 其北部的北华北坳陷中的内黄隆起、东濮凹陷的电性结构相对简单, 为低阻、高阻2层结构, 与该区地壳由沉积盖层和基底整体性较好的坚硬地块结晶基底组成相对应。新乡-商丘断裂以南的南华北坳陷中的构造单元深部的电性结构相对复杂, 纵向上表现为低-高-低3层结构, 横向上高阻、低阻交互出现, 分析其可能与受到扬子板块与华北板块南缘相互俯冲碰撞影响有关。 而北华北坳陷因边界断裂--新乡-商丘断裂的调控, 受到扬子板块和秦岭-大别造山带造山作用的影响较小。1342年和1918年河南通许2次破坏性地震的震源位于太康隆起下方的高、低阻边界交接区域, 1737年河南封丘5^1/2级地震的震源位于壳内高、低阻梯度带附近。

关键词: 华北坳陷, 电性结构, 新乡-商丘断裂, 地震成因