A look at galvanic distortion in the Tasman Sea and the Juan de Fuca plate

We have examined magnetotelluric (MT) data from the Juan de Fuca plate and the Tasman Sea to understand the influence of galvanic distortion by local and regional topography. Galvanic (non-inductive) distortion, caused by the build up of charge along conductivity gradients, is present in both regions. Its effects can be removed by decomposition of the impedance tensor assuming that both electric and magnetic field distortion are present. Unlike for most land-based data, magnetic field distortion is necessary to explain the seafloor data. Electric field distortion parameters resulting from the full decomposition can be interpreted equivalently in terms of either 2-D or 3-D distorting bodies, but these interpretations cannot be differentiated using MT observations alone. The data are compared to the overall geology to determine whether 2-D or 3-D distortion is more probable. The electric field distortion parameters in both locations behave appoximately frequency-independently, as expected for galvanic distortion. When interpreted in terms of 2-D distorting bodies, the superficial strikes were found to parallel the trends of coastlines and large-scale submarine features (e.g. mid-ocean ridges). This suggests that distortion both in the Tasman Sea and on the Juan de Fuca plate is dominantly galvanic and appears to be caused by regional, large-scale, 2-D features rather than small-scale, 3-D distorting bodies.