Enlarge / It may not look like much, but that red line marks the Gales Creek fault. (credit: Horst et al./BSSA) Coastal denizens of the US Pacific Northwest are (or should be!) familiar with the significance of “the Big One”—a major earthquake just off the coast that will occur someday. The tectonic plate boundary, where oceanic crust slides downward beneath the North American continent, is capable of producing major shaking and an incredibly dangerous tsunami.