Summary
Lying off the Pacific Northwest, where a plate of ocean crust dives beneath North America and into the mantle, the Cascadia subduction zone is best known for a mammoth magnitude 9 earthquake in 1700 that sent a tsunami to Japan. But in modern times, it has been ominously quiet, with almost none of the small, daily earthquakes that are common at other subduction zones. Stress building up at the fault seemingly has had no release. Now, a 2-month cruise of the premier U.S. academic seismic imaging ship is capturing Cascadia’s structure in fine detail, hoping to answer questions about whether the fault is segmented, the shallowness of past earthquake slips, and the properties of the thick layer of sediments coating the plates. Answers from the cruise will eventually inform seismic hazard projections throughout the region.