Geologists have warned that another powerful earthquake could inflict terrible damage on Tokyo because the Size 9 monster which struck on March 11 has altered the earth's surface.
The quake has put pressure on the fault lines near the Japanese capital and experts have suggested that a size 7.5 magnitude earthquake could hit.
The structure of the tectonic plates and fault lines around the city makes it unlikely that Tokyo, home to 13million people, would be hit by a quake anywhere near the intensity of the one 10 days ago, said Roger Musson of the British Geological Survey.
But given the vast population - the capital and its surroundings are home to 39million people (including in the suburbs) - any strong tremor could be devastating.
'Even if you've got, let's say, a 7.5, that would be serious,' the seismologist said.
Japan is located on the Ring of Fire, an arc of volcanoes and fault lines spanning the Pacific Basin, and is regularly hit by earthquakes.
But before the last quake - the largest to hit the country since it started keeping records 130 years ago - few geologists considered Japan to be a strong candidate for a 9-plus earthquake, said Andrew Moore, of Earlham College in Richmond, Indiana.
There is mounting evidence, however, that Japan has been struck by several severe quakes in the last 3,500 years - most in the northern reaches of the country.
Sand deposits indicate that several quakes have spawned 30-foot-high (9-metre-high) waves that slammed into the northern island of Hokkaido, he said, the most recent in the 17th century.
Similar deposits underlie the city of Sendai - the area rocked last week - with the most recent from an 869 A.D. tsunami that killed 1,000 people and washed more than 2.5 miles (three kilometres) inland.
And even weaker quakes that hit Tokyo in the past have caused significant damage.
Earthquakes are common in Japan, one of the world's most seismically active areas.
The country accounts for about 20 per cent of the world's earthquakes of magnitude 6 or greater.
Several continental and oceanic plates - the Pacific Plate, Philippine Plate, Eurasian Plate and North American Plate - meet in the Japan area, which is why there are so many volcanoes and hot springs across the nation.
Located in a volcanic zone so active it is nicknamed the Pacific Ring of Fire, catastrophic earthquakes occur several times each century.
Japan has suffered an estimated 200 recorded tsunamis in its history due to earthquakes that take place below or close to the Pacific Ocean.
In October 2004, an earthquake with a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 people and injuring more than 3,000.
That was the deadliest quake since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400.
But the tremor from March 11 changed the coastal landscape - and not just above sea-level.
It created a trench in the sea floor 240 miles long (380 kilometres long) and 120 miles wide (190 kilometres wide) as one tectonic plate dove 30 feet (nine metres) beneath another, said Eric Fielding of NASA's Jet Propulsion Laboratory.
While that relieved stress at the breaking point, it appears to have piled pressure on to adjacent segments, said Brian Atwater, a geologist with the U.S. Geological Survey.
That added strain could now trigger a strong, deadly aftershock on Tokyo's doorstep.
It's a common occurrence after strong quakes and happened after the 2004 mega-earthquake and tsunami off Indonesia that killed 230,000 people in a dozen nations.
Three months later, an 8.6-magnitude quake erupted farther down the fault line, killing 1,000 people on sparsely populated Nias island.
'But it's difficult to say,' said Atwater. 'There are good examples of such stresses leading to other earthquakes, big earthquakes, and there are good examples of that not happening.'
Scientists are studying the March 11 quake and ongoing seismic activity to determine where new strains might be building.
'When the main shock is this big, you get a football-shaped region where aftershocks are fair game. It extends in all directions,' including toward Tokyo, USGS seismologist Susan Hough and other experts said.
But, they acknowledge, it's hard to keep up.
Ross Stein, of the USGS, said: 'We are drinking from a fire hose here. The input data keeps changing and augmenting.'
His focus now is on the fragment of the Pacific tectonic plate lodged beneath Tokyo - movement of which is believed to have caused a 7.3-magnitude quake in 1855 that killed an estimated 7,000 people.
'We believe the faults which bound the fragment were brought closer to failure by the magnitude-9 quake,' Ross said.