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

本研究计划为期三年，旨在对第四纪活火山分布广泛的中国东北地区和俄罗斯滨海边疆区的深部地幔电导率分布进行详细的研究。为此，我们与中国和俄罗斯科学家合作，对研究区的天然电磁场变化进行了长期观测。本文采用近年来发展起来的大地电磁测深（MT）网络方法，对2001年吉林省大地电磁测深网的电场变化数据和长春地磁场数据进行了分析，并反演得到了一维（仅与深度有关）电导率模型。所得的电导率剖面进行了比较，从北美和夏威夷的其他研究。结果表明，上地幔和下地幔最上部的电导率与过去的结果相比没有显著差异，而地幔过渡带的电导率比过去的结果高出近一个数量级。初步报告发表在《地球物理研究快报》（Ichiki et al.，2002年，网络机器翻译实验在辽宁省南迁。利用长春地区3条测线的电场数据和几个周期的磁场数据得到了响应函数，并分别反演成一维电导率剖面。结果表明，在地幔过渡带，电阻率略高。然而，得出的结论是，有必要在更广泛的区域进行更多的实验，以解决这种差异是否只是明显的或反映了在深度的电导率的横向变化。这一结果将发表在《地球物理研究快报》上。

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.