LOS ANGELES — As dozens of aftershocks continued to rattle Japan, scientists said Tuesday they were worried that Friday's magnitude 9.0 temblor might trigger a dangerous earthquake close to Tokyo, the largest urban center in the world.
The fear is that the initial quake and the series of large aftershocks that follow will transfer geophysical stresses into nearby faults, causing some near Tokyo to shift violently, said Michael Wysession, a seismologist at Washington University in St. Louis.
Already, the pattern of aftershocks in Japan appears to be shifting southward from waters off the coast of Sendai to Tokyo, which is 231 miles away. On Tuesday night local time, a magnitude 6.2 quake struck near Shizuoka, 72 miles southwest of the Japanese capital.
That quake "upped the ante" for Tokyo, said Thomas Jordan, director of the Southern California Earthquake Center at the University of Southern California.
Concerns about the quake risk in the Tokyo metropolitan area — home to 32.5 million people — seem to rise from multiple sources.
First, large earthquakes like the one that struck near Sendai last week spawn hundreds of aftershocks along nearby faults and plate boundaries. Some of these aftershocks can be very powerful in their own right. In Japan, aftershocks have been as high as 7.1.
Second, Tokyo is situated at a "triple junction," where three tectonic plates — in this case, the Pacific, the Philippine and the Eurasian plates — come together. Such busy intersections are often seismically active and "tend to produce a lot of fracturing and breaking off of stuff," said Ross Stein, a U.S. Geological Service geophysicist.
"Tokyo is in the center of everything, and it's pathologically located in harm's way," he added.
And then there's the fact that this week's aftershocks "seem to be migrating from Sendai toward Tokyo," said John Rundle, a seismologist at the University of California at Davis .
In 2008, a study that assessed the seismic risk in Tokyo concluded that the city faces a 30 percent chance of a quake with shaking as intense as that felt in Sendai in the next 30 years.
Friday's quake appears to have "slightly increased the stresses on faults around Tokyo," said Stein, who led the study. Still, he emphasized that exactly what it might herald is unclear.
Since the 1970s, seismologists have been anticipating a Big One of at least a magnitude 8.0 that could have devastating consequences for Tokyo. They already have a name for it: the Tokai quake.
Still, scientists noted that predicting earthquake activity was always a tricky business. "We have an incredible habit of being wrong," Stein said.