A massive 8.9 magnitude earthquake – the fifth-largest since 1900 -- struck at 2:46 p.m. local time Friday (12:46 a.m. ET), centered about 100 miles east of Sendai on Japan’s main island, Honshu.
The quake generated a tsunami, between 23 and 32 feet, that swept boats, cars, buildings and debris miles inland. Smaller swells struck other Pacific Rim countries, causing relatively minor damage.
The country is located in the "Ring of Fire" arc of volcanoes and oceanic trenches partly encircling the Pacific Basin. About 20 percent of the world's earthquakes of magnitude 6.0 or greater hit Japan. Tokyo, with a population of 12 million, sits on the junction of four tectonic plates: the Eurasian, North American, Philippine and Pacific.
Alan Boyle writes on his blog ( Science editor at msnbc.com,)
This week's earthquake caused the main island of Japan to shift as much as 13 feet to the east, seismologists say. That may sound like a shocker, but it's just one of the natural changes that come along with an 8.9-magnitude temblor — like the 1.6-microsecond speed-up of Earth's daily rotation and the 4-inch shift in Earth's axis.
The eastward shift was documented by Japan's Geonet network of GPS monitoring stations, based in Tsukuba, said Ken Hudnut, a geophysicist at the U.S. Geological Survey's Earthquake Hazards Program in Pasadena, Calif. Similar shifts took place during last year's 8.8 earthquake off the Chilean coast, as well as the 9.1 earthquake near Sumatra that caused a disastrous tsunami in 2004.
"It's the same phenomenon in all three cases," Hudnut said. The movement is linked to the release of the strain that builds up when one tectonic plate grinds against another in a subduction zone.
"What's going on is that the plate going down drags along with it the upper plate as strain is stored in between earthquakes," he explained. "When the earthquake occurs, the upper plate lurches eastward over the subducting plate. The oceanic plate that's going down is relatively rigid, but the upper plate is like a wedge of material that's more elastic. So picture that upper wedge as being almost like an accordion that's being compressed between the times of earthquakes. It's like a spring. You're loading up the spring between earthquakes — in other words, you're compressing the eastern edge of the spring toward the main island of Japan. The earthquake allows that material to spring out toward the east."
