Is it time for the Philippine Plate to get active?
Also, the Japanese researchers have gone very quiet on the subject, says BBC's Richard Black and Jonathan Amos.
From BBC News (4/21/2011):
...Seismic activity has definitely increased since the M 9.0 event, with the incidence of small earthquakes registered in some parts of Japan, hundreds of kilometres from the source, increasing by a factor of 10.
Hazard assessment in the region is therefore a pressing priority for researchers; but it is far from easy.
"The Kanto region is very complex, and the size of quakes triggered there is probably going to be limited by that complexity," says Chris Goldfinger from Oregon State University in the US, who works in collaboration with Japanese researchers.
"But given the proximity to Tokyo, even a limited quake there would be damaging."
Kanto sits very close to a tectonic triple junction - a point where vast slabs of the Earth's surface grind past each other.
The tectonic plate making up the Pacific Ocean floor is moving westwards under Japan towards Eurasia.
The Pacific plate is being pulled down (subducted) underneath Japan; and crowding in on this collision is the Philippine Plate, further south, also trying to get under Japan.
Ross Stein from the US Geological Survey (USGS) is one of a US/Japanese team that has modelled the region around the triple junction to help gauge future risks.
They used seismic signals from 300,000 tremors of various sizes to build a three-dimensional view of what was going on deep in the Earth, much like a doctor might use X-ray tomography to scan tumours in the brain.
They found a 25km-thick fragment broken off one of the plates that they now believe plays a significant role in shaping seismic activity in the Kanto region - and by implication, the outlook for Tokyo.
"When it comes to Tokyo, things get immensely complicated," said Dr Stein.
"There seems to be broken pieces of plate that are jammed under Tokyo like a pill that won't go down your throat. And on top of that we have the two different slabs of plates beneath it, so there's really a triple stack of faults beneath Tokyo."
To make matters more complicated, there is some disagreement among researchers about the most important geological factors around Tokyo, with some pointing to the Sagami Trough (a subduction zone leading off the triple junction) as the likely cause of big earthquakes, and others citing Dr Stein's "pill", known as the Kanto Fragment.....
(They've been probably told by the Japanese government not to talk without permission.)
One curiosity is that Japanese researchers are currently very reluctant to talk on the issue.
Scientists we contacted - in one case, someone known to one of us for years - did not want to go on record.
Given the devastation caused by the tsunami and the fact that an event near Tokyo could do even more damage, the reluctance to talk is eminently understandable.
It can, however, be interpreted as a signal of real concern.
The BBC article has an audio of the interview with Dr. Stein of USGS.