Here in the Pacific Northwest, there’s a risk no one wants to think about: “The Really Big One,” a magnitude 9 earthquake along a 700-mile stretch of coast — from Mendocino in Northern California to Vancouver Island in Canada — followed by waves reaching 100 feet high. Seismologists call it the Cascadia subduction zone, where the oceanic Juan de Fuca Plate moves to the east and slides below the much larger, mostly continental North American Plate.