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.”
Robert Geller, a geophysicist at the University of Tokyo who is a critic of scientists who try to predict quakes, said he wasn’t alarmed by the new report: “From a scientific point of view, it may be true the risk is slightly higher than normal, but the increase is not marked enough that any special precautions are warranted.”
Monday’s aftershock, centered 100 miles northeast of Tokyo and measuring magnitude 6.6, wasn’t nearly as powerful as the 7.1-magnitude tremor four days earlier, but it was strong enough to knock out electricity for 50 minutes at the nuclear plant in Fukushima.
Another earthquake, this one magnitude 6.2, struck early Tuesday in Japan, just 47 miles east of Tokyo. It sparked a fire near one of the Fukushima plant’s reactors.
The incidents did not appear to have any safety implications, an official from Japan’s Nuclear and Industrial Safety Agency said. But more people are being evacuated from their homes. Government spokesman Yukio Edano said the risks of a widespread radiation leak had become “considerably lower,” but he said that, because of radiation readings, additional areas within 12 to 19 miles of the plant will be given mandatory evacuation orders.
During the past month, the aftershocks have decreased in number and will continue to do so. But scientists say the range of magnitudes does not change, and thus a large aftershock, or even another “main shock” on a nearby fault, is possible.
Hanging over Japan is the worrisome example of Indonesia. Three months after the Dec. 26, 2004, magnitude-9.1 quake that generated a catastrophic tsunami, the adjacent segment of the fault broke again, this time in a magnitude-8.7 earthquake. The fault system has since generated several more powerful earthquakes.
But that pattern was not repeated after major quakes in Haiti and Chile. The absolute rule of earthquakes seems to be that there are no absolute rules. And earthquake science has been humbled by recent events that did not fit the consensus of what was likely. The Tohoku earthquake, for example, was far stronger than most scientists thought possible on that section of the trench.
What isn’t debatable is that all of Japan is vulnerable to earthquakes, and Tokyo is in a particularly dicey location. Tokyo sits on the gentle Kanto Plain, adjacent to a large bay that is protected by a peninsula from the battering effects of tsunamis and typhoons. But there are faults in every direction and a triple-junction of tectonic plates just offshore where slabs of Earth meet, grind and sometimes violently lurch past one another.
Stein is among those who say that a rogue slab of tectonic plate is wedged below the Kanto Plain — like a piece of debris from all the geological collisions.
Tokyo last suffered a devastating earthquake in 1923. The Great Kanto Earthquake triggered fires that raced across the city and created a firestorm that immolated tens of thousands of people taking refuge in a field.
After the March 11 quake, Stein prepared a letter, authorized by his superiors, that offered basic advice to American officials in Japan: Carry a whistle, water, food, a first-aid kit, flashlight and batteries, work gloves and a trowel for digging people out of debris. Look around for objects that might become lethal missiles in an earthquake. Identify the sturdiest table in the room — and be ready to dive under it when the next big one hits.
Staff writer Chico Harlan in Tokyo contributed to this report.