The U.S. Geological Survey revised its estimate of the strength of the earthquake north of Japan, now saying it had a magnitude of 8.3. But tsunami warnings and watches -- which covered the Pacific basin from Washington State all the way to Vietnam -- were canceled after seismologists said the threat had passed.
The earthquake, centered in the Kuril Islands, which stretch northeastward from Japan's northernmost major island of Hokkaido, was strong enough to be labeled a "great" quake on the scale used by the U.S. earthquake-hazards program.
But the Kuril Islands are sparsely populated, and there were few reports of damage from the region.
A tsunami can happen if an earthquake causes a violent shift on the ocean floor -- pushing large amounts of water out of the way. That's what happened off the coast of Indonesia in December 2004, when walls of water more than 30 feet high inundated shorelines around the Indian Ocean, killing at least 213,000 people in 11 countries.
The initial earthquake that day was measured at a magnitude of 9.1 -- considerably larger than today's. The scale for earthquakes is geometric -- a magnitude-9 quake, for instance, is 10 times as violent as a magnitude-8 quake.
Today the largest wave reported was just 16 inches high, reported at the northern Japanese port of Nemuro, about three hours after the earthquake occurred.
A few minutes later, a second, 8-inch wave hit the nearby port city of Kushiro, where 4,000 people had initially been ordered to evacuate to higher ground.
The quake struck at 6:14 a.m. ET (8:14 p.m. in Japan) in the open waters of the Pacific, about 1,000 miles northeast of Tokyo. The nearest island is known as Etorfu, 245 miles to the east.
The quandary for scientists is that they can quickly tell the strength of an earthquake, but they cannot immediately tell what it has done to the sea floor. So on a day like today, they post "tsunami watches," indicating that conditions might be right for a tsunami, even if there is no indication that one has formed.
The U.S. has Tsunami Warning Centers in Hawaii and Palmer, Alaska. Both posted watches right after the first earthquake report but soon took them down.
The Associated Press contributed to this report.