The earthquake that struck off the Japanese island of Honshu on Friday created classic conditions for a powerful and destructive tsuanmi, geological experts told ABC News.
"It's the largest earthquake we believe has taken place off western Honshu since good seismological recording began in the early 20th century," said Dr. John Ebel, director of the Weston Observatory of Earth and Environmental Sciences at Boston College.
He said the thrust quake occurred when one tectonic plate—the Pacific Ocean plate—slid under another, the Asian plate. That type of earthquake is called a subduction quake—the same kind of quake that hit Sumatra in 2004, causing a horrific tsunami, and that caused devastation in Chile last year.
The 8.9-magnitude quake on Friday met three conditions to create a killer tsunami, according to Eric Calais, professor of geophysics at Purdue University. It was a shallow earthquake, occurring not that far into the earth's crust. Its epicenter was offshore, and it created a motion that raised the sea floor.
"It's a giant wave-making machine," said Dr. Larry Brown, professor of earth and atmospheric sciences at Cornell University.
All of Japan—in the famous Ring of Fire, an arc of volcanoes and earthquakes zones in the Pacific--is highly vulnerable to earthquakes, experts said, because the islands are located on a major boundary between the two tectonic plates. "It is THE dangerous geologic setting because it's capable of producing monster earthquakes up to and beyond a magnitude of 9," Calais said.
Seismic energy is building up over time in the entire region, he said. "The only way the earth can release that buildup of energy is by snapping from time to time." The energy release of this earthquake, Brown said, is 1,000 times greater than the energy released by the explosion of the atomic bomb in Hiroshima in 1945.
Japan takes all the steps possible to minimize earthquake losses, experts say. "They are the world's best-prepared country," says Brown.Their building codes are strict, seawalls line vulnerable coast areas, and skyscrapers are constructed to sway in the event of earthquakes.
But earthquake forecasting is notoriously difficult. Scientific focus in Japan has been centered on the area closer to Tokyo, said Ebel, and the disaster Friday will renew discussion. "It raises the stake a little bit in terms of the threat to Tokyo. This will now be the big debate: can we have an earthquake of this size further south?"
In the short run, aftershocks are inevitable, scientists say. More than 80 with a magnitude above 5 have already been felt, and aftershocks with a magnitude above 7 are very possible. "Those are large events by themselves and can continue for months and years," says Calais. Some could create more tsunamis, but they would be smaller than the torrents that hit Japan today. But it is difficult to predict if they would be destructive, he said.