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 pr esent in both regions. Its effects can be removed by decomposition of the impedance tensor assuming that both electric and magnetic field di stortion are present. Unlike for most land-based data, magnetic field distortion is necessary to explain the seafloor data. Electric field d istortion parameters resulting from the full decomposition can be inte rpreted equivalently in terms of either 2-D or 3-D distorting bodies, but these interpretations cannot be differentiated using MT observatio ns alone. The data are compared to the overall geology to determine wh ether 2-D or 3-D distortion is more probable. The electric field disto rtion parameters in both locations behave appoximately frequency-indep endently, as expected for galvanic distortion. When interpreted in ter ms of 2-D distorting bodies, the superficial strikes were found to par allel 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 t o be caused by regional, large-scale, 2-D features rather than small-s cale, 3-D distorting bodies.