The earthquake that shook Japan in March, triggering a tsunami that left more than 24,000 people dead or missing, was bigger than seismologists expected and may indicate another large temblor closer to Tokyo is possible, U.S. researchers said.
Before the 9-magnitude quake on March 11, the buildup of tension along a fault had suggested a smaller tremor, scientists led by Mark Simons, a professor of geophysics at the California Institute of Technology in Pasadena, wrote yesterday in Sciencexpress, the online version of the journal Science.
The so-called Great East Japan Earthquake happened on a fault known as the Japan Trench megathrust, where the Pacific Plate sinks below the plate on which Japan sits at about 8 centimeters (3 inches) a year, the researchers said. During the quake, some parts of the Pacific Plate slipped as much as 60 meters, which is as much as three times more than the temblor that rocked Chile in February last year.
“The extent to which the 2011 earthquake was unexpected suggests that we should consider the potential for similar large events elsewhere on the Japan Trench megathrust,” Simons and colleagues wrote. “An earthquake similar to the 2011 event is possible offshore Ibaraki and Fukushima just south of the most recent event.”
Sea Mountain
The researchers hypothesized that a mountain on the seabed of the sinking plate may have prevented it from slipping for as long as 1,000 years, leading seismologists to the false conclusion that it was sliding slowly under the Japan plate without building up quake-generating tension. Instead, the mountain may have been limiting the amount of slip on neighboring areas and accumulating tension, they wrote.
An area south of where the quake occurred that hasn’t ruptured may hold the potential for another large temblor, the researchers wrote.
Scientists should make “a concerted effort to determine the extent to which this area is stuck with the potential for generating a large earthquake or whether it is creeping, thereby posing less of a threat,” Simons said in an e-mail.
A 6-magnitude earthquake struck off the coast east of Tokyo at 9:46 a.m. today, according to the U.S. Geological Survey. It was the biggest aftershock to hit the country since May 14, when a 6.2-magnitude temblor struck further north.
Japanese superquake moved ocean floor 79 feet sideways and 10 feet up - and new data shows region is under more strain
By JANE BUNCE
The ocean floor shifted sideways by 79 feet in the Japanese earthquake in March - much further than scientists originally predicted.
And researchers are warning that immense amounts of seismic stress remain stored in the area, putting it at risk of further devastating earthquakes.
The journal Science has published three new papers about the effects and causes of Japan's March 11 mega-quake, which paints a picture of an earthquake hot spot much more complex and potentially dangerous than scientists had ever anticipated.
The tsunami comes crashing towards Japan's Fukushima Daiichi nuclear complex. Researchers say the area is at risk of further serious quakes
In one paper, the Japanese Coast Guard has released data from five geodetic instruments that in 2000-04 they had placed underwater along the fault line responsible for the colossal quake.
One of the instruments had actually been placed almost on top of the epicentre of the Magnitude 9.0 quake, at a station called MYGI.
Measurements taken in the week following the earthquake showed that at the MYGI site, the sea floor had moved about 79 feet to the east-southeast since the previous measurement in February. It had also risen about 10 feet.
Dr Mariko Sato, a geodesist with the Japan Coast Guard in Tokyo, believes almost all this movement happened during the quake.
Ships sitting on a rice field amid rubble from the tsunami. Scientists believe the disaster moved the sea floor much further than originally thought
'The scale is almost double that estimated only from the terrestrial data,' Dr Sato told BBC News.
Under the seabed, the movement may have been even greater - perhaps 160 to 200 feet, by some estimates.
In another study sure to raise alarm in Japan, scientists from the California Institute of Technology have reconstructed how the Tohuku-Oki earthquake unfolded using GPS data recorded at more than 1,200 sites.
Their data showed that - contrary to previous opinion - the area had built up massive amounts of strain prior to the earthquake.
Earlier, there had been general agreement among researchers that the 'Miyagi segment' of the fault line was not under the stress of other segments along the Japan plate boundary, where large earthquakes occur at a regular basis. But Professor Mark Simons' team showed that this assumption was deeply flawed.
This raises questions about other sections of the fault line that had previously been considered low risk - including areas further south, closer to Tokyo.
This 'Ibaraki segment' of the plate boundary has been thought to behave in similar fashion to that of the Miyagi segment, and Professor Simons says it may likewise hold large amounts of seismic stress.
In recorded history, this southern area has experienced only one set of quakes larger than magnitude 8 - which means the region could be ripe for its own rupture.
The quake may also have destablised nearby areas of fault line, making them even more vulnerable to a catastrophic rupture.
'We have to entertain the possibility this area can produce a large quake,' Simons said. 'This area will warrant a lot of attention in the near future.'
The earthquake and subsequent tsunami left more than 24,000 people dead or missing, and wiped out entire towns.
