Enormously large tippers observed in South-West of China. Can realistic 3-D modeling reproduce them?

Vertical magnetic transfer functions (tippers) observed at a global/continental net of geomagnetic observatories can be used to image the electrical conductivity structure of the Earth down to a depth of around 200 km. We estimated tippers at 54 geomagnetic observatories across China, aiming eventually to invert them in terms of subsurface three-dimensional (3-D) conductivity distribution. Strikingly, we obtained enormously large tippers at three inland observatories in southwest China. Large tippers are often observed at coastal observatories due to the high lateral conductivity contrast between resistive continental bedrock and conductive seawater. However, tippers at those mentioned above observatories appeared to be a few times larger than coastal tippers. Moreover, as far as we know, such large tippers (reaching value 3) were never reported in any region of the world. We perform 3-D electromagnetic simulations mimicking the geological setting of the region and demonstrate that the enormously large tippers are feasible and can be attributed to a current channelling.