Geomagnetic depth sounding analysis of the electrical conductivity of the mid-mantle beneath Taiwan

Abstract The electrical conductivity structure of the mid-mantle at depths of 300–900 km beneath Taiwan was investigated for the first time by applying geomagnetic depth sounding (GDS) to geomagnetic field data. These data were obtained by six Taiwanese observatories and three observatories outside Taiwan. We refine the conductivity estimates by addressing distortions due to the ocean induction effect, which is caused by the contrast in electrical conductivity between the highly resistive continental crust and highly conductive seawater and oceanic sediments. To accurately correct for this effect, we employed a high-resolution (0.1° × 0.1°) three-dimensional conductivity model in a spherical coordinate system with locally refined horizontal meshing. Simulations indicated that the ocean induction effect could best be reproduced using a value of 100 Ω m for the crustal resistivity on land, which is significantly lower than values used in previous global studies. The corrected GDS responses were then inverted to obtain one-dimensional electrical conductivity–depth profiles for each of the six Taiwanese observing sites. The average conductivity profiles showed an increase in conductivity with depth, from 0.06 S m −1 at 300 km to 0.86 S m −1 at 900 km. In the upper mantle, the south-eastern part of Taiwan is more conductive than the north-western part. At greater depths, the conductivity values were in line with those in global models, indicating a more homogeneous lower mantle structure. The results of this study enhance the understanding of mantle dynamics beneath Taiwan and demonstrate the critical role of precise ocean effect corrections in GDS analysis. Graphical Abstract