A new calculation by American and Japanese scientists has concluded that the March 11 event might have heightened the stress on faults bracketing the ruptured segment of the Japan Trench.
“There’s quite a bit of real estate on which stress has increased, by our calculations,” said U.S. Geological Survey geophysicist Ross Stein. “The possibility of getting large, late aftershocks to the north and south of the main shock is real.”
Stein and two colleagues, including lead author Shinji Toda of Kyoto University, are not making a formal prediction of another big earthquake. But their research paper, which has been submitted to the journal Earth, Planets and Space, contends that the magnitude-9.0 “Tohoku earthquake” (Tohoku is the region in northern Japan closest to the epicenter) shifted stress to the north and south, including onto a section of the Japan Trench east of Tokyo.
“That section of the subduction zone is clearly loaded,” said Chris Goldfinger, an Oregon State University seismologist who was not part of the new research.
“It will take probably a decade before this aftershock sequence is over,” Stein said. “The watchword in Tokyo should be long-term vigilance. Nobody should think this should go away in a few weeks or a few months.”
The earthquake was so violent that the northern portion of the main island of Honshu lurched more than three feet to the east. Global Positioning System measurements showed places where the surface moved more than 16 feet. There have been more than 200 aftershocks.
Although scientists continue to search for ways to anticipate an imminent tremor, for now there is nothing like a practical system of earthquake prediction. Earthquakes appear to be spawned in a fundamentally chaotic manner. No one can say when or where an earthquake will happen, or how strong the shaking will be.
The new calculation is not based on direct measurements of the accumulated stress among the faults that surround the rupture of March 11. Rather, the scientists looked at the aftershocks and concluded that they supported the hypothesis of stress being unloaded by the main event onto adjacent faults.
“Stress relieved during an earthquake does not simply dissipate; instead it moves down the fault and concentrates in sites nearby, promoting subsequent tremors,” the authors write.
This is a field not known for consensus. Two scientists said Monday that, even if there is some increase in stress on nearby faults as a result of the March 11 event, it is probably minimal and surely difficult to quantify.
“Big earthquakes don’t cascade like dominoes — bang, bang, bang. At least, not commonly,” said Susan Hough, a USGS geologist who has written about earthquake prediction. “I think the maps showing bright red bull’s-eyes of increased stress may be more alarming than they should be.”