As dozens of aftershocks continued to rattle Japan on Tuesday, scientists said they were worried that Friday's magnitude 9 earthquake might trigger a dangerous temblor close to Tokyo, the largest urban center in the world.
The fear is that the initial quake and the series of large aftershocks will transfer geophysical stress 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 toward Tokyo from off the coast of Sendai, 231 miles away. On Tuesday night local time, a magnitude 6.2 quake struck near Shizuoka, 72 miles southwest of the capital.
That quake "upped the ante" for Tokyo, said Thomas Jordan, director of the Southern California Earthquake Center at USC.
Concern about the quake risk in the Tokyo metropolitan area, home to 32.5 million people, seems to arise from multiple sources.
First, large earthquakes spawn hundreds of aftershocks along nearby faults and plate boundaries. Some of these aftershocks can be very powerful. In Japan, aftershocks have been as big as 7.1.
Second, Tokyo is situated at what's known as a triple junction, where three tectonic plates — in this case, the Pacific, the Philippine and the Eurasian plates — come together. Such 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 in Menlo Park, Calif.
"Tokyo is in the center of everything, and it's pathologically located in harm's way," he said.
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 UC Davis.
A 2008 study that assessed the seismic risk in Tokyo concluded that the city faced a 30% 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 that could have devastating consequences for Tokyo. They already have a name for it: the Tokai quake.
But the Japan Meteorological Agency said Tuesday that the Shizuoka aftershock was not directly related to Tokai, said Shinji Toda, a geologist at Kyoto University, who has worked closely with Stein.
The Shizuoka aftershock was a strike-slip quake, which occurs when two sides along a fault move sideways relative to each other. The Tokai quake would be a megathrust quake, when one tectonic plate is pushed beneath another, Toda said.
Scientists noted that predicting earthquake activity was always a tricky business.
"We have an incredible habit of being wrong," Stein said.