The quake happened at the intersection of the North American and Pacific plates in the northwestern chunk of the "Ring of Fire," in an area that "has been incredibly quiet," Applegate said.
Japan's worst quake in modern times was a magnitude-8.3 in 1923 in Kanto that killed 143,000 people, according to the USGS. A 7.2-magnitude quake in Kobe in 1995 killed 6,400 people. That Kobe quake surprised experts because there had not been a megaquake in the Kobe region in modern times, said Kathleen Tierney, director of the Natural Hazards Center at the University of Colorado.
Image: People stranded at a building since Friday's powerful earthquake-triggered tsunami, are evacuated on boats in Ishimaki, Miyagi Prefecture, northern Japan on March 13, 2011.