关键词: 电磁测深方法, 电性结构特征, 三河-平谷地震

Abstract: An electromagnetic array profiling(EMAP) and magnetotelluric sounding (MT) have been conducted in the Sanhe-Pinggu M8 earthquake area, China. The EMAP profile is 31.8km in length, while the total length of two MT profiles is 150.05km.The profiles reveal the principal features of the electrical structures of the studied area. The relation of all these features to the development and occurrence of strong earthquakes is discussed based on the analysis of the principal characteristics of shallow and deep electrical structures. The electrical structures of Sanhe-Pinggu M8 earthquake area are characterized by its significant layering, and the area as a whole can be classified into 7 electrical layers. A high conductivity layer can be recognized at a certain depth of the electrical structures of the Sanhe-Pinggu area. The depth of the central part of the high conductivity layer is about 20km,and the average thickness of the layer is about 11.5km. Several gradient zones of abrupt variation of resistivity were observed within the high conductivity layer, and it is believed that they might bear a certain relation to the nucleation and occurrence of strong earthquakes. Anomalous bodies of various scales with very low resistivity have been found along both the NE-trending and NW-trending observation traverses in the studied area. The resistivity of these bodies is one magnitude less than that of the high conductivity layer. They appear as isolated blocks, instead of layers. There is no direct link between the deep-seated and surficial faults inferred from the variation of the electrical structures. This may indicate the differences and discontinuity of the shallow and deep-seated structures. The lithosphere thickness is thicker in mountainous region and thinner in plain region. The depth of the lower boundary of lithosphere in this area varies from 70km to 94 km.

Key words: Magnetotelluric Sounding, Electrical structures, Sanhe-Pinggu earthquake