A study on the mantle electrical conductivity structure beneath the eastern edge of continental Asia

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This three-year research project aimed to make a detailed investigation of the electrical conductivity distribution in the deep mantle beneath region including the NE district of China and the Primorsky region of Russia, where active Quaternary volcanoes widely distribute. For this purpose we carried out long-term observation of natural electromagnetic field variations in the study area in cooperation with Chinese and Russian scientists. Network-MT method developed by the present investigators were employed to reveal the deep structure in the mantle.In 2001, electric field variation data from the network in Jilin province and magnetic field data at Changchun were analyzed and inverted into one-dimensional (depth only dependent) conductivity model. Resulting conductivity profiles were compared to those of other studies from North America and Hawaii. It was found that no significant difference exists in the conductivity of the upper mantle and the uppermost lower mantle between the present and past results, while the mantle transition zone of the present result showed higher conductivity by nearly one order of magnitude. A preliminary report was published in Geophysical Research Letters (Ichiki et al., 2001).In 2002, Network-MT experiment was shifted to the south in Liaoning province. Response functions were also obtained by using electric field data from 3 lines and magnetic field data at Changchun up to the period of several, and were individually inverted into one-dimensional conductivity profiles. The results were compared with profiles from Jilin province, and were found to be slightly more resistive in the mantle transition zone. However, it was concluded that additional experiments in wider area are necessary to resolve whether this difference is only apparent or reflects lateral variation of conductivity at depths. This result is going to be published in Geophysical Research Letters.