Chapter 1

Pacific Northwest magnetic survey

Scripps Institution of OceanographyScripps Institution of OceanographyScripps Institution of Oceanography, San Diego, California was established to support marine biological research, but from 1908 onwards pioneered geophysical research in the oceans. Teddy Bullard was a frequent visitor and undertook early ocean floor heat flow measurements with Roger Revelle. In 1962 the University of California established the new Institute of Geophysics and Planetary Physics alongside Scripps in La Jolla. acquired its first seagoing ship in 1937 and by 1951 had four modern seagoing research ships and more to come. In 1961 a Scripps expedition made a magnetic survey of the seafloor off the Pacific Northwest of the US, showing symmetrical patterns of magnetic anomalies, offset along what appeared to be faults. On one of them, a very obvious feature, the offset was not clear. “Victor Vacquier went out later, trying to find the offset by running magnetic profiles on either side”, recalls McKenzie. “The problem was that the offset was more than 1000 kilometres, which was why they couldn’t find it earlier.” This was a problem: the fault ran up to the US continental margin and stopped, with no apparent effect. “No one could understand what they meant. They also had no idea how the magnetic stripes were produced.”

IMAGE REFERENCE: Raff’s & Mason’s index anomaly map of the Pacific off the Northwest coast of the US (1961) [originally from: Raff A D & Mason, R G, “Magnetic Survey off the west of North America 40° N Latitude to 52° N Latitude”, Geological Society of America  Bulletin, v72 (1961), pp1267-1270], but reused as a figure for McKenzie, D. & Julian, B., 1971. The Puget Sound, Washington, earthquake and the mantle structure beneath the northwestern United States. Bulletin of the Geological Society of America. 82: 3519-24.