later pull the supercontinent Pangaea apart. As this successor to Rodinia split, the Atlantic Ocean was born 200 million years ago. But before it became a proper ocean, it was a network of narrow rift valley lakes—not unlike today’s East African Rift Valley—and it invited prehistoric crocodiles and dinosaurs to its shores. But the division also left behind a bedrock riven with the geological equivalent of stretch marks. Rumbles, like the one that cracked the Washington Monument in 2011, remind us that ancient scars exist unseen all along the east coast, but for the most part, these old faults are quiet. To get earthquakes you need not only faults, but strain.

In California there’s plenty of strain. Where the Pacific Plate grinds inexorably against the edge of North America,New Madrid a slew of forces act on the 600 million-year-old Reelfoot Rift, and come from all directions, converging here in the middle of the continent. First, from below. There’s an odd little tectonic fragment off the coast of the Pacific Northwest, called the Juan De Fuca plate. It’s all that remains of a once gigantic slab of Pacific Ocean seafloor that began diving under North America during the Cretaceous. Today this fragment is responsible for the inevitable mega-quake (outlined in Kathryn Schulz’ Pulitzer Prize-winning New Yorker story last year) that will pummel Seattle. But the rest of this formerly enormous ocean plate is now mostly underneath the United States, having slid under the continent for tens of millions of years. Far underneath the middle of the country,