The geomagnetic response across the continental margin off Vancouver Island: Comparison of results from numerical modelling and field data.

Abstract

Geomagnetic variation data have been measured along a profile from central Vancouver Island, across the continental shelf to the deep seafloor on the Juan de Fuca plate. As expected, the maximum response in the geomagnetic coast effect occurs near the shelf-edge and the fall-off inland and seaward is observed. The direction and magnitude of the real part of the induction arrows vary slightly with period between 30 and 103 minutes. The induction arrows are co-linear at all sites and are aligned approximately normal to the shelf-break trend. At the seafloor sites, the imaginary part of the induction arrows has a large variation with period, reversing its direction at periods less than 35 minutes. At the land sites, the imaginary part increases in magnitude with increasing period.The responses of several numerical models were calculated using the finite difference algorithm of Brewitt-Taylor and Weaver. The comparison of the calculated and observed responses shows that a good conductor is required at sub-lithospheric depths beneath the profile. The addition of a conductive wedge of sediments beneath the continental shelf improves the fit. The best-fitting model incorporates this wedge and a conductive, shallow-dipping slab representing the Juan de Fuca plate subducting under Vancouver Island.