Magnetotellurics has been used to study of the geo-electric structure of the Olympic Mountains and Puget Lowland. Data were collected along an west-east line extending from the Pacific Ocean to the Cascade Mountains. The smoothest resistivity model consistent with the data suggests that subduction of the Olympic Mountains accretionary complex (Core rocks) has been prevented by a 30+ km thick sequence of volcanic rocks. This may effectively couple the Juan de Fuca and North American Plates and may explain the concentration of crustal earthquakes beneath the Puget Lowland. ¿ 1998 American Geophysical Union