The geomagnetic coast effect in the Pacific Northwest of North America

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Abstract
During the summer of 1984, Brown University measured magnetic variations at a series of 9 sites along an 80 km profile which began in the Willamette Valley in West‐Central Oregon, extended westward across the Coast Range, and terminated at the coast of the Pacific Ocean. Our primary objective was to investigate the "coast effect" with a density of coverage and at shorter periods (10‐100 sec) than had previously been attempted in this region. The data collected show a significant coast effect at sites closest to the ocean. While it is tempting to assume that, to first‐order, all such effects are due to induced electric currents flowing in the ocean, our studies for the Oregon coast show that the contribution of electric currents flowing in the lithosphere, especially the contribution of induced currents in a thick accretionary wedge of marine sediments offshore, is actually more significant than the contribution from the ocean itself. In addition, we have detected a conductive feature beneath the Coast Range which appears to be another zone of structurally coherent sediments along our profile. The pattern of short‐wavelength induction anomalies inland suggests that at these periods a very close spacing of sites (5 km or less) is called for in future studies.