08/06/2025
The Cascadia subduction zone runs for 700 miles off the coast of the Pacific Northwest, beginning near Cape Mendocino, California, continuing along Oregon and Washington, and terminating around Vancouver Island, Canada. If the southern part of the subduction zone gives way, the magnitude of the resulting quake will be somewhere between 8.0 and 8.6. “That’s the big one,” Kathryn Schulz writes. 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.”
When the next very big earthquake hits, the northwest edge of the continent will drop by as much as six feet and rebound 30 to 100 feet to the west—losing, within minutes, all the elevation and compression it has gained over centuries. Some of that shift will take place beneath the ocean, displacing a colossal quantity of seawater. The water will surge upward into a huge hill, then promptly collapse. One side will rush west, toward Japan. The other side will rush east, in a 700-mile liquid wall that will reach the Northwest coast, on average, 15 minutes after the earthquake begins. By the time the shaking has ceased and the tsunami has receded, the region will be unrecognizable. Kenneth Murphy, who directs FEMA’s Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, says, “Our operating assumption is that everything west of Interstate 5 will be toast.”
In the Pacific Northwest, the area of impact will cover some 140,000 square miles, including Seattle, Tacoma, Portland, Eugene, Salem (the capital city of Oregon), Olympia (the capital of Washington), and some seven million people. When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America, outside of the 2010 Haiti earthquake. “This is one time that I’m hoping all the science is wrong, and it won’t happen for another thousand years,” Murphy said. “In fact, the science is robust,” Schulz notes. Revisit her 2015 report on the devastating earthquake that could happen within the next 50 years: https://www.newyorker.com/magazine/2015/07/20/the-really-big-one?utm_source=facebook&utm_medium=social&utm_campaign=dhfacebook&utm_content=app.dashsocial.com%2Fnewyorkermag%2Flibrary%2Fmedia%2F561862324
