The Russian earthquake this week was a big one: the sixth most powerful in recorded history. But it was not the Big One, the quake that so many Californians fear will one day rip across the San Andreas Fault. Nor was it the Really Big One, the earthquake that scientists predict will someday devastate the Pacific Northwest, killing nearly thirteen thousand people, according to one FEMA estimate. That quake was the subject of a 2015 New Yorker investigation by the writer Kathryn Schulz, who won a Pulitzer Prize and a National Magazine Award for her reporting. The Really Big One, Schulz warned, could compromise as many as a million buildings across the region, and may very well become the worst disaster in North American history….

‘Take your hands and hold them palms down, middle fingertips touching. Your right hand represents the North American tectonic plate, which bears on its back, among other things, our entire continent, from One World Trade Center to the Space Needle, in Seattle. Your left hand represents an oceanic plate called Juan de Fuca, ninety thousand square miles in size. The place where they meet is the Cascadia subduction zone. Now slide your left hand under your right one. That is what the Juan de Fuca plate is doing: slipping steadily beneath North America. When you try it, your right hand will slide up your left arm, as if you were pushing up your sleeve. That is what North America is not doing. It is stuck, wedged tight against the surface of the other plate.

Without moving your hands, curl your right knuckles up, so that they point toward the ceiling. Under pressure from Juan de Fuca, the stuck edge of North America is bulging upward and compressing eastward, at the rate of, respectively, three to four millimetres and thirty to forty millimetres a year. It can do so for quite some time, because, as continent stuff goes, it is young, made of rock that is still relatively elastic. (Rocks, like us, get stiffer as they age.) But it cannot do so indefinitely. There is a backstop—the craton, that ancient unbudgeable mass at the center of the continent—and, sooner or later, North America will rebound like a spring. If, on that occasion, only the southern part of the Cascadia subduction zone gives way—your first two fingers, say—the magnitude of the resulting quake will be somewhere between 8.0 and 8.6. That’s the big one. If the entire zone gives way at once, an event that seismologists call a full-margin rupture, the magnitude will be somewhere between 8.7 and 9.2. That’s the very big one.…’ –Kathryn Schulz via The New Yorker