By Richard Black and Jonathan Amos
Scientists are trying to establish if the Magnitude 9.0 Tohoku earthquake has altered the chances of a major tremor under Tokyo - or increased the risk of another tremor powerful enough to generate a tsunami.
The massive Sumatra quake in 2004 was followed by many others above Magnitude 7.0, including two above Magnitude 8.0 in 2005 and 2007.
Some generated tsunamis that claimed more than 100 lives; and it is thought they occurred because the original earthquake, on 26 December, increased stresses along the tectonic plate boundary that lies to the west of Sumatra and Java.
So what is the outlook for Japan now, especially for the great city of Tokyo and the Kanto plain on which it sits?
This is home to one-quarter of Japan's population, as well as being the country's administrative and commercial centre.
Big quakes struck the area in 1703, 1855 and 1923, with the latter claiming the lives of 100,000 people.
Were any one of these events to occur today, the economic losses alone would be expected to top $1 trillion (80 trillion Yen).
Tokyo is the administrative and commercial heart of Japan. 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.
On-going calculations
The USGS team has tried to work out the probabilities of repeat quakes like those in 1703, 1855 and 1923.
The Magnitude 9.0 Tohoku earthquake is one of the biggest in recorded history
Prior to the Tohoku, the group figured Tokyo had a 30% chance in the next 30 years of experiencing an event (M 7.0-plus) that produces the sort of severe shaking which would test even the walls and pillars of highly earthquake-resistant buildings.
The question now facing the group is whether this assessment needs to be revised following the M 9.0 Tohoku tremor and its aftershocks - the biggest of which, very close to Tokyo, registered M 7.9 just 30 minutes after the big one.
"We're hard at work making just the calculation you suggest," Dr Stein told the BBC.
"We calculate that there are modest increases in stress on some of the faults that lie just to the south of Tokyo.
"So our judgement would be that the hazard is either unchanged or higher than it was beforehand."
But Dr Stein stresses that far more work needs to be done.
Compared with the situation in Sumatra, researchers are at least fortunate that there is an abundance of data available from seismic monitoring stations on land and at sea. The region is well-studied and well-modelled.
As Dr Stein puts it: "We are drinking from a fire-hose of data about earthquake occurrence."
However, surprises do occur. Chris Goldfinger says the size of the Tohoku quake was itself a reminder that understanding of subduction zones is incomplete.
"[The Tohoku zone] had been written off as a really great seismic source," he told BBC News.
"It was well known to put out quakes at M 8.3-8.4 quakes, but on the seismic hazard maps it was lightly treated - all the hazard was thought to be on the Nankai Trough [on the boundary of the Eurasian and Philippine plates].
"But it surprised everyone; and that's why I no longer write off faults unless they're proven dead."
'Outer-rise'
The USGS modelling also suggests an increase in earthquake risk in northern Honshu, in the Sanriku region.
Scientists have access to a huge amount of data
It also shows stresses increasing to the east of the Japan Trench.
Here, the Pacific Plate is distorted into a ridge as it approaches the subduction zone down which is must eventually travel.
Parts of these ridges can collapse suddenly in what are known as "outer rise" events, which can generate a tsunami.
The Kuril Islands to the north of Japan saw an outer rise earthquake in 2007 - which reached M 8.1, and generated a tsunami.
The biggest concern, however, is Tokyo.
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.
When the models are complete and peer-reviewed - and Dr Stein's is not the only one in existence - we should have a clearer view of the situation.
In the meantime, all we can be sure of, he says, is that the 30% risk he calculated four years ago has certainly not gone down.
Jonathan.Amos-INTERNET@bbc.co.uk
BBC News
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