The magnitude 9.0 earthquake that unleashed the devastating 2011 tsunami in Japan was triggered by the largest fault slip ever recorded, the journal Science reported Thursday.
By measuring the frictional heat produced by the fault slip during the earthquake, researchers from the University of California, Santa Cruz and other organizations found that friction along the Tohoku fault was remarkably low when the earthquake struck on March 11, 2011.
"The Tohoku fault is more slippery than anyone expected," said Emily Brodsky, a geophysicist at UC Santa Cruz. Brodsky acted as co-author for three papers on the Tohoku earthquake published in the journal Science this week.
Researcher Patrick Fulton, first author of the paper focusing on temperature measurements, concurred.
"The large slip at shallow depths contributed to the tsunami that caused so much damage in Japan. Usually, these earthquakes don't rupture all the way to the surface," he said.
Fulton said that the low resistance to slip along the Tohoku fault can help explain the staggering 165-foot displacement, or movement, that occurred to the seafloor during the earthquake. That low friction, he said, was exacerbated by an abundance of weak, slippery clay material in the fault zone.
The clay reportedly resists slippage only slightly better than a banana peel. This clay is abundant along the subduction zone in the Pacific Ocean where the Tohoku earthquake occurred. Subduction zones are the the boundaries between tectonic plates. In this case, the Pacific plate is driven beneath the Eurasian plate east of Japan.
In the deep portion of the fault, where the two plates are "locked" together, a lot of tension or stress is built up over time. Geologic tests have revealed that the frictional stress on the shallow portion of the fault was very low during the earthquake, which the researchers say means that either the stress was low to begin with or all of the stress was released during the earthquake.
"It's probably a combination of both - the fault was pretty slippery to begin with, and whatever stress was on the fault at that shallow depth was all released during the earthquake," Fulton said.
This week's three Science papers, along with another published in the same journal in February of this year, also suggest that a near-total stress drop occurred during the earthquake
"We now have four lines of evidence that frictional stress was low during the earthquake," Brodsky said in a statement. "The key measure is temperature, but those results are totally consistent with the other papers."
The research not only sheds light on the forces that caused the Tohoku earthquake, but also has implications for earthquake studies in the future.
"We've been hamstrung without in situ measurements of frictional stress, and we now have that from the temperature data," Brodsky said. "It's hard to say how generalizable these results are until we look at other faults, but this lays the foundation for a better understanding of earthquakes and, ultimately, a better ability to identify earthquake hazards."
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