Seafloor magnetotelluric sounding above axial seamount

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Abstract
Axial Seamount is a large, active, ridge axis volcano located on the central segment of the Juan de Fuca Ridge in the northeast Pacific Ocean. Magnetotelluric (MT) data have been collected at three sites, approximately 4 km apart around the eastern rim of the volcano, during a 65‐day deployment. MT responses, in the bandwidth of 10²–105 s, are almost isotropic, with a weakly‐defined principal direction of strike parallel to the main topographic trends of Axial Seamount, and are relatively flat over the whole bandwidth. Apparent resistivities are of the order of 7–20 Ωm, and phases are as low as 30° at the short periods. Diagonal terms of the MT tensor are an order of magnitude smaller than the off‐diagonal terms, suggesting that three‐dimensional effects on the data are minimal. Two‐dimensional inversions suggest that seafloor bathymetry and the distant coastlines have a surprisingly small effect on the MT response, and one‐dimensional inversions fit the MT data to within the errors with no serial correlation in the residuals. A low crustal resistivity is the most robust part of the model, probably due to seawater in fractures and possibly due to a magma chamber. An electrical asthenosphere, although less well constrained, exists over a depth range 30–60 km, and the resistivity of this region is compatible with about 8% fraction of melt.