March 11, 2011 at 1:34 PM ET
Seismic experts have long known that Japan’s complex undersea fault system is capable of unleashing great waves, but today's 8.9-magnitude quake was the most violent shock to hit the nation in the past century. And tsunami geologist Jody Bourgeois of the University of Washington was there to feel it for herself, on the Japanese island of Hokkaido.
"The shaking didn't seem really strong, but it just kept going," she told me today during a Skype phone call. "I felt like I was seasick, which was really strange."
Bourgeois is working with other seismic experts this winter at the University of Hokkaido in Sapporo, about 300 miles north of the quake's epicenter, and she's been keeping track of Japan's recent spate of seismic activity — including a 7.2 quake that struck off the Japanese coast on Wednesday.
"I was sitting at my desk today, not really thinking we're going to get another earthquake, and then the room started to move," she said. "I didn't hear any people suggest that the 7.2 might be a foreshock. ... But now you realize that those were just the forerunners."
The quake, which ranks as the world's fifth-strongest seismic event of the past century, was centered near the southern end of an undersea subduction zone that extends from Russia's Kamchatka Peninsula and the Kuril Islands to the Japanese islands. Such a zone, where one tectonic plate dives beneath another, is a classic generator of tsunami waves.
"Japan's very complicated because they've got several different plates coming together here," Bourgeois explained.
Specifically, the quake occurred "in the subduction zone of the Japan Trench where the Pacific Plate subducts under the Honshu island of Japan, part of the Okhotsk Plate, at a rate of about 8 centimeters per year," Jayanta Guin, senior vice president of research for AIR Worldwide, said in an advisory.
Quake history goes back centuries
Paul Caruso, a geophysicist at the U.S. Geological Survey, said the quake occurred in one of the world's most seismically active regions. "That area has had nine earthquakes above magnitude 7 since 1973," he said.
An undersea quake measuring 8.2 to 9.0 in magnitude occurred in the same subduction zone off the coast of Kamchatka in 1952, generating a Pacific-wide tsunami that caused considerable damage and loss of life in Russia. Japan's deadliest quake, a 7.9 shock that killed more than 140,000 people in 1923, occurred south of today's epicenter on the main island of Honshu.
Going farther back, Bourgeois said core samples indicate that Japan's Sendai Plain was hit by a strong earthquake and tsunami back in the year 869. Some experts believe this was the strongest seismic disturbance to hit Japan in recorded history.
The same area, including Sendai's airport, was hit the hardest by today's quake. "It's been 1,200 years since they had one this large," Bourgeois said.
Other danger zones
Bourgeois said the same kind of offshore subduction zone was implicated in the magnitude-8.8 earthquake that did so much damage a year ago. A stronger subduction seaquake, pegged at magnitude 9.1, caused the tsunami that swept through the Indian Ocean in 2004, causing more than 200,000 deaths around the Pacific Rim.
Just last year, seismologists reported that the Bourgeois' home base — the Pacific Northwest — is vulnerable to the same kind of earthquake and tsunami. The 680-mile-long Cascadia fault has been dormant for 300 years, and seismologists say there's an 80 percent chance that the portion of the fault off southern Oregon and northern California could produce a megaquake in the next 50 years.
Aftershocks have been continuing in Japan, more than 12 hours after the 8.9 shock occurred, and they may continue for days or weeks longer. "We can expect a lot of aftershocks," Caltech seismologist Kate Hutton told MSNBC.
Bourgeois doesn't intend to stay holed up in her office. "I know at least one group that's going out tomorrow to the coast," she said. But she also doesn't intend to get in the way.
"We go out basically after a tsunami, when people are safe and you're not going to interfere with the local rescue efforts," Bourgeois said. "We're interested in everything — what kind of damage the tsunami did, how high the currents were. I'm interested in what kind of record the tsunami left behind, and Japan is very well set up for these kinds of surveys."
More on the Japan quake and tsunami:
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