Americans have long dreaded the “Big One,” a magnitude 8.0 earthquake along California’s San Andreas Fault that could one day kill thousands of people and cause billions of dollars in damage. The Big One, though, is a mere mini-me compared with the cataclysm forming beneath the Pacific Northwest. Roughly 100 miles off the West Coast, running from Mendocino, California, to Canada’s Vancouver Island, lurks the Cascadia Subduction Zone, where the Juan de Fuca Plate is sliding beneath the North American Plate, creating the conditions for a megathrust quake 30 times stronger than the worst-case scenario along the notorious San...