A tsunami warning in Japan is raising questions. Here’s how tsunamis form, why warnings change and what people far from the coast should know.

On Monday, at 4:53 P.M. local time, a magnitude 7.7 earthquake struck off the northeastern shores of Japan’s largest island, Honshu, where the Pacific tectonic plate plunges beneath the North American...

No abnormalities were reported at the Fukushima Daiichi, Fukushima Daini and other nuclear power plants, their operator said.