A tsunami, which in Japanese means “harbor wave,” is sparked by a strong motion on the ocean floor, usually by an earthquake, a volcanic eruption or an underwater landslide.
In Japan’s case, there was an earthquake where chunks of the Earth’s crust separated under the seafloor, causing the massive, 8.9-magnitude quake.
Earthquakes cause not just a single wave but a series that acts like the waves rippling from a stone dropped in a pond. Each wave can last from five to 15 minutes.
Experts say the subsequent waves are hard to predict because it’s difficult to know how an earthquake has affected the seafloor until hours, days or sometimes months after the event.
And the impact of tsunamis is hard to detect on the open ocean until they reach full speed, which happens only when they near the shore.