Sounding Out Beneath the Sea for the Next ‘Big One’ in the Pacific Northwest

By Jim Brace-Thompson

Deep down far beneath the waves of the ocean lapping the Pacific Northwest is a gash some 810 miles long referred to as the Cascadia subduction zone. There, the Juan de Fuca Plate of oceanic crust (along with the much smaller associated Explorer Plate to the north and Gorda Plate to the south) is taking a deep dive beneath the North American Plate. As it does so, it has generated huge stratovolcanoes across northern California, Oregon, Washington, and British Columbia—Mount St. Helens, Mount Rainier, Mount Hood, and Mount Shasta, to name but a few.

The plate boundary also generated one of the greatest recorded earthquakes in North America back in 1700. Clocking in at an estimated magnitude 9.0, it not only shook up the Pacific Northwest but also sent a tsunami rolling clear to Japan!

However, in recent times quakes here have been few and far between. “It’s just way, way, way too quiet” down there, says marine geologist Chris Goldfinger (Oregon State University) as reported in the July 2, 2021, issue of the journal Science. Earth scientists aboard the research vessel Marcus G. Langseth hope to learn why.

On a two-month mission, the ship is currently zig-zagging the coast with an airgun, blasting sound waves down into the crust and capturing echoes via hydrophones and other receivers positioned both on the ocean floor and inland along the coast. Those echoes are helping earth scientists and seismologists map the underground terrain of the Cascadia subduction zone at an unprecedented level of detail.

If all goes well this summer, by the end of the voyage of the Langseth, we should have plenty of data to help map the Cascadia subduction zone to help shed light on subduction zone dynamics in general and to help tell us: Is the next Big One coming?