Photograph by Sankei via Getty Images
Published December 5, 2013
The largest fault slip ever recorded produced the devastating 2011 Japan tsunami, according to three studies published today.
Two years ago, the sea off the coast of Japan reared up and swept away tens of thousands of lives in a devastating natural disaster.
Now, three papers published today in the journal Science reveal the magnitude 9 earthquake off the east coast of Japan still has the capacity to surprise.
Experts calculate the fault—or the boundary between two tectonic plates—in the Japan trench slipped by as much as 164 feet (50 meters). Other similarly large magnitude earthquakes, including the 9.1 Sumatra event in 2004, resulted in a 66-to-82 foot (20-to-25 meter) slip in the fault.
"We've never seen 50-meter [slips]," said Kelin Wang, a geophysicist with the Geological Survey of Canada in British Columbia.
The next largest slip would probably be the Chile earthquake in 1960, said Wang, who was not involved in the research. Based on the limited data recorded from that earthquake, the fault slipped by 98 to 131 feet (30 to 40 meters).
Most of the movement occurred horizontally, he explained. But because the plates are wedged together at this trench, that horizontal displacement still managed to thrust up enough seawater to produce the killer tsunami that hit Japan.
Greasing The Wheels
Lubrication, specifically involving clay, is the key to such massive movement, said Frederick Chester, a geophysicist at Texas A&M University in College Station, and lead author of one of the studies.
The two tectonic plates involved are the Pacific plate, on which the Pacific Ocean resides, and a portion of the North American plate, on which parts of Japan sit.
A thick layer of clay sits atop the Pacific plate, which is getting dragged under a portion of the North American plate. As the Pacific plate dives into a trench off the coast of Japan, small portions of the clay get smeared along the plate boundary, Chester explained.
That clay traps water, rendering it quite slippery, he said. "We think that's responsible for allowing the incredibly large slip we observed near the trench."
Normally, when two plates collide, there is friction. You can think of friction like a brake, Chester explained. "But clay almost removes any braking properties."
Not a Lot of Heat
The unprecedented data haul that enabled Chester and colleagues to figure out what happened during the 2011 earthquake is courtesy of a rapid response by the Japan Agency for Marine-Earth Science and Technology, said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and a co-author of one of the studies.
The Japan Trench Fast Drilling project enabled researchers to get out to the fault zone about a year after the earthquake and drop instruments down to measure temperature anomalies—the sudden slips during an earthquake can generate vast amounts of heat—and to bring up samples of the fault zone itself for analysis.
Experts were able to take core samples of sediment and rock from the trench—located in 23,000 feet (7,000 meters) of water—thanks to a sophisticated drilling ship.
"[This] was right at the edge of what engineering could do," Brodsky said.
Not only did they find evidence of this thin layer of lubricating clay, but experts were also able calculate how much heat and friction was involved.
Even though the earthquake produced a 1,100° to 2,200°F (600° to 1,200°C) temperature increase, the amount of friction that had to be overcome to produce the fault slip wasn't as large as researchers expected, said Brodsky.
This helped confirm the fact that something else was going on—namely the clay lubrication.
It's difficult to say whether something like this could happen elsewhere, said Wang, because no other submarine trench has as many instruments monitoring it.
"Nowhere else do we have such a massive monitoring system."
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Andrew Allison I didn't see a lot of letters behind your name indicating that you have any education or expertise in geology. Could you please enlighten us as to how you came to such a judicial conclusion that the article was "rubbish"?
This nonsense beggars belief. If there were a lubricative layer between the plates, they would slide, relatively gently, rather than suddenly slip 50 meters. The San Andreas fault in California, which is actually the most highly instrumented fault in the world, and which is sliding incrementally in central California while jerking in the north and south is instructive in this regard.
Can you correct your post? Japan is not on the North American Plate. It sits at the boundary between 3 plates, but none of them are North American. Phillipine, Eurasian, and Pacific are more accurate.
Absolutely fascinating. Been following the whole story since March 12 '11, thanks so much NatGeo for printing this story. Hope much more will follow.
@Maks Tarn so clever yes they are
Not necessarily. San Andreas is by no means the golden standard--when you're looking at something at this order of magnitude, it becomes staggeringly complex, and there's no guarantee (and no reason to assume) that every plate boundary moves smoothly. The Pacific Rim, especially near Japan, is a very complex series of faults, which impacts how pressure builds at different locations. Big earthquakes happen in places where patches of the plates get stuck against each other and build tension for years/decades/centuries, and sometimes these lubricating factors exacerbate the resulting quakes when they do finally slip.
This is the cause of the historically large 9+ quakes in Chile, Alaska, and the Pacific Northwest (all of which are generally around the same Pacific Rim plate, but all of which behave very differently from San Andreas). This is also the cause of the prevalence of volcanoes ringing the Pacific Ocean (the Ring of Fire)--those are regions where the Pacific Plate (or a smaller plate that's chipped off the Pacific Plate) is subducting below another plate. It's all the same system, but with radically different behaviors and outcomes in different regions.
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