On a map of Japan that shows seismic hazards, the area around the prefecture of Fukushima is colored in green, signifying a fairly low risk, and yellow, denoting a fairly high one.
Most scientists expected the next whopper to strike the higher-risk areas southwest of Fukushima, which are marked in orange and red.
"Compared to the rest of Japan, it looks pretty safe," said Christopher H. Scholz, a seismologist at the Lamont-Doherty Earth Observatory at Columbia University, referring to the area hit worst by the quake on March 11. "If you were going to site a nuclear reactor, you would base it on a map like this."
Sometimes, scientists are blindsided by earthquakes because they occur along undiscovered faults. The deadly earthquakes in New Zealand this year; in Haiti last year; in Northridge, Calif., in 1994; and in Santa Cruz, Calif., in 1989 all happened along faults that scientists were unaware of until the ground shook.
"It's shameful, but we've barely scratched the surface," said Ross Stein, a geophysicist with the United States Geological Survey. In California, for instance, scientists have cataloged 1,400 faults, yet for smaller earthquakes — magnitude 6.7 or less — about one in three still occur on previously unknown faults.
"Humbling," Dr. Stein said.
That raises a worrisome question: How many major quakes are lurking in underestimated or unknown faults?