In May, the federal government simulated an earthquake so massive, it killed 100,000 Midwesterners instantly, and forced more than 7 million people out of their homes. At the time, National Level Exercise 11 went largely unnoticed; the scenario seemed too far-fetched — states like Illinois and Missouri are in the middle of a tectonic plate, not at the edge of one. A major quake happens there once every several generations.

But Tuesday’s earthquake along the East Coast is a reminder that disasters can hit where they’re least expected. And if the nightmare scenario comes, government officials w...