A huge tsunami spawned by the magnitude-8.9 quake—one of the largest ever recorded—rocked Japan's eastern coast, killing hundreds of people, according to the Associated Press.

(Get the inside story of how nations rally to aid others after earthquakes, tsunamis, and other natural disasters.)

Within hours, the tsunami hit Hawaii and set off warnings throughout the Pacific, including South America, Canada, Alaska, and the U.S., such as the Oregon coast, the AP reported. (See more tsunami pictures.)

• A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. More rarely, a tsunami can be generated by a giant meteor impact with the ocean.

• Scientists have found traces of an asteroid-collision event