Scientists warn of years of aftershocks in Japan, and risks on other faults

Japan won't stop shaking. One month after the horrific March 11 earthquake and tsunami, the island was rattled anew by aftershocks: A magnitude-6.6 quake on Monday was followed by a 6.3 quake on Tuesday.

Monday's quake was strong enough to knock out electricity briefly at the damaged Fukushima nuclear plant.

Four days earlier, a magnitude-7.1 quake led to four deaths and widespread power outages. With soldiers still looking for the bodies of thousands of people who vanished a month ago, Japan is coping with the painful reality that it sits in a seismic bull's eye.

Now scientists are warning that the March 11 event not only will lead to years of aftershocks but also might have increased the risk of a major quake on an adjacent fault. A new calculation by American and Japanese scientists concluded that the March 11 event heightened the strain on a number of 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 Ross Stein, a U.S. Geological Survey (USGS) geophysicist. "The possibility of getting large, late aftershocks to the north and south of the main shock is real."

Stein and two Japanese colleagues, including lead author Shinji Toda of Kyoto University, have submitted their research to the journal Earth Planets Space. The scientists are not making a formal prediction of another big earthquake. But they believe the section of the Japan Trench east of Tokyo now has more stress than before March 11.

"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.

The processes that lead to earthquakes are too chaotic to be predictable in any practical sense. Two other scientists said that even if there is an increase in seismic hazard on nearby faults, it is minimal and hard to quantify.

Susan Hough, a USGS geologist who has written extensively on the subject of earthquake predictions, sounded a skeptical note when asked about the increased risk of a big quake: "Big earthquakes don't cascade like dominoes, bang bang bang. At least not commonly. So I think the maps showing bright red bull's eyes of increased stress may be more alarming than they should be."

Hanging over Japan is the worrisome example of Sumatra. Three months after the Dec. 26, 2004, magnitude-9.1 earthquake that generated a catastrophic tsunami, the adjacent segment of the same fault broke again, in a magnitude-8.7 earthquake. The fault system has since generated several more powerful earthquakes.

"It will take probably a decade before this aftershock sequence is over," Stein said.

Tokyo is in a particularly treacherous location. It sits on the gentle Kanto Plain, adjacent to a large bay that is protected by a peninsula from Pacific Ocean tsunamis and typhoons. But there are faults in every direction, and a triple-junction of tectonic plates just offshore where slabs of the earth meet, grind and sometimes violently lurch past one another. Beneath the Kanto Plain might be a rogue slab of crust, debris from all these 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.