Laboratory electromagnetic model results for the EMSLAB region

Abstract

Apparent resistivities and vertical to horizontal magnetic field ratios for the Lincoln Line traverse in the EMSLAB project region are studied using a scaled laboratory analogue model. The model includes a simulation of the Juan de Fuca Plate subducting the Canadian‐U.S. Pacific Northwest region. In the model, the subducting plate is taken to be horizontal offshore, dipping at 10° beneath the coast, then inland bending farther to dip at 45° in the Washington‐Oregon region and at 30° in the British Columbia region. The model results show that for inland sites the apparent resistivity for the telluric field component parallel to the coast, and thus also parallel to the strike of the dipping conductive substructure, depends strongly on period, while the resistivity for the telluric field perpendicular to the coast is largely a galvanic effect and changes less with period. The amplitude of the vertical to horizontal magnetic field ratio along the Lincoln Line shows a saddle shaped response, with the coastal maximum followed by a second maximum inland due to the dipping substructure. Further, rather than decreasing rapidly with distance inland as expected for a simple coast effect, the field ratios retain significant values large distances inland for periods of 3–120 min. Model results for 120‐min period are compared with one example of field results for the EMSLAB project region.