Measurement of network - MT in two areas of NE China for study of upper mantle conductivity structure of the back - Arc region

The upper mantle electrical conductivity structure of the Pacific back - arc beneath northeastern China was investigated by the long - period magnetotelluric (Network - MT ) at Changchun city and Sanchahe site. Time series data of the long baseline electric field were collected by using land telephone line cables and three component magneic field variations were recorded at Changchun observatory by digital flux magnetometer. Network - MT responeses have been obtained up to 10⁵-10⁶s, which were inverted to one - dimensional model down to depths of about 1 000km. Resulted models show that: (1) The discontinuity were detected at 100-200km, and in mantle transition zone for around 300 km and 500-600 km depth, respectively. (2) The discontinuity appears at 850km for the lines in Changchun. (3) The conductivity almost monotonically increases with depth in the smooth conductivity models. (4) The conductivity reaches 1 S/m at 600 km depth in Sanchahe sites and at 850 km in Changchun sites, respectively. The above results indicate the discontinuity and the heterogeneity of the mantle and can be compared with the results obtained by petrology studies.