For emergency planners, there’s a simple maxim: “If you’re ready for an earthquake, you’re ready for a lot of different things,” Barb Graff (no relation), the head of emergency management in Seattle, once told me. But for her in Seattle, it’s also a necessity: The most real threat the Pacific Northwest faces is a megathrust earthquake along what’s known as the Cascadia Subduction Zone.
California’s “Big One,” along the San Andreas, gets most of the attention, but there are three other U.S. faults that cause emergency planners perhaps even more heartburn.
First, the Cascadia Subduction Zone, a fault about 700 miles off the Pacific Coast of Oregon and Washington, or what the New Yorker called in 2015, “The Really Big One.” The fault, when it goes, might unleash “only” an earthquake between 8.0 and 8.6 magnitude, which itself would rank as one of the most powerful and destructive quakes to ever hit the United States. But a so-called “full margin rupture” of the fault would prove truly catastrophic, potentially topping 9.0. Beyond the quake’s damage from the shaking, it could cause a multi-hundred-mile-long tsunami to inundate the west coast…
