Geologists have warned that an earthquake could strike near Tokyo after the recent disaster which hit north-eastern Japan altered the Earth’s surface, putting stress on a segment of the fault line near the city.
Japan braced for second earthquake
Roger Musson from the British Geological Survey said the structure of the tectonic plates and fault lines around Tokyo means it is unlikely that the city will be hit by a quake of the same intensity as March 11’s 9.0 episode.
But given how densely populated Tokyo is - the capital and its surroundings house more than 39M people - Musson explained that any strong tremor could be devastating.
He said: “Even if you’ve got, let’s say, a 7.5, that would be serious.”
The country is on the Ring of Fire, which is an arc of volcanoes and fault lines across the Pacific Basin, and it is regularly hit by earthquakes.
But before last week’s 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 9m high waves which 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 AD tsunami which killed 1,000 people and washed more than 3km inland.
And even weaker quakes that hit Tokyo in the past have caused significant damage.
But last week’s tremor changed the coastal landscape - and not just above sea level. It created a trench in the sea floor 380km long and 190km wide as one tectonic plate dived 9m 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 US Geological Survey (USGS).
That added strain could now trigger a strong, deadly aftershock on Tokyo’s doorstep.
It is a common occurrence after strong quakes and happened after the 2004 mega-earthquake and tsunami off Indonesia, which killed 230,000 people in a dozen nations.
Three months later, an 8.6-magnitude quake erupted further 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 towards Tokyo, USGS seismologist Susan Hough and other experts said.
But, they acknowledge, it is hard to keep up.
“We are drinking from a fire hose here. The input data keeps changing and augmenting,” Ross Stein, of the USGS, wrote in an email.
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,” Stein said.