Fukushima Nuclear Plant at High Risk for Major Earthquake

Map of Japan's islands indicating the area of study (black box). The purple star marks the epicentre of the 11 March earthquake and the red star the Iwaki epicentre. Fukushima Daiichi is highlighted by a red square. Black triangles indicate active volcanoes. Numbers on the side of the image represent latitude and longitude. (Credit: Ping Tong, Dapeng Zhao and Dinghui Yang)

Fukushima Nuclear Plant at High Risk for Major Earthquake

Scientists issue call for safeguards to prevent another nuclear disaster

A new study shows that last year's catastrophic earthquake in Japan caused a seismic fault close to the Fukushima nuclear plant to reactivate, meaning another major earthquake could strike the area again, this time even closer to the Fukushima plant.

The new information (pdf) is published in European Geosciences Union's journal Solid Earth.

The March 11, 2011 earthquake's epicenter was about 160 kilometers from the Fukushima Daiichi nuclear power plant. The scientists in the study explain that new tremors are happening even closer to the site:

"There are a few active faults in the nuclear power plant area, and our results show the existence of similar structural anomalies under both the Iwaki and the Fukushima Daiichi areas. Given that a large earthquake occurred in Iwaki not long ago, we think it is possible for a similarly strong earthquake to happen in Fukushima," says team-leader Dapeng Zhao, geophysics professor at Japan's Tohoku University.

The 11 April 2011 magnitude 7 Iwaki earthquake was the strongest aftershock of the 11 March earthquake with an inland epicentre. It occurred 60 km southwest of the Fukushima nuclear power plant, or 200 km from the 11 March epicentre.

The research now published in EGU's Solid Earth shows that the Iwaki earthquake was triggered by fluids moving upwards from the subducting Pacific plate to the crust. The Pacific plate is moving beneath northeast Japan, which increases the temperature and pressure of the minerals in it. This leads to the removal of water from minerals, generating fluids that are less dense than the surrounding rock. These fluids move up to the upper crust and may alter seismic faults.

"Ascending fluids can reduce the friction of part of an active fault and so trigger it to cause a large earthquake. This, together with the stress variations caused by the 11 March event, is what set off the Iwaki tremor," says Ping Tong, lead author of the paper.

The number of earthquakes in Iwaki increased greatly after the March earthquake. The movements in the Earth's crust induced by the event caused variations in the seismic pressure or stress of nearby faults. Around Iwaki, Japan's seismic network recorded over 24,000 tremors from 11 March 2011 to 27 October 2011, up from under 1,300 detected quakes in the nine years before, the scientists report.

"We think it is possible for a similarly strong earthquake to happen in Fukushima"

The scientists in the study report that since an earthquake near the Fukushima plant is likely, more attention should be paid to safeguards to prevent another nuclear disaster.

* * *

Meanwhile at the stricken plant, agencies report that the temperature at the No.2 reactor continue to rise to dangerous levels, though TEPCO says a faulty thermometer gauge may be the reason:

The Tokyo Electric Power Company said the temperature in one of the damaged reactors at its Fukushima nuclear station rose above safety limits yesterday, even as it injected increased amounts of cooling water.

One of three thermometers indicated the temperature at the bottom of the No.2 reactor pressure vessel rose to 93.7 degrees, , exceeding the 80 limit, said Ai Tanaka, a spokeswoman for the utility known as Tepco.

There are no signs of isotopes that would suggest the reactor has gone critical and there has been no increase in radiation around the site, the company said in a statement. The other two thermometers at the bottom of the vessel showed temperatures of 32.8 and 33.1 earlier yesterday, spokesman Naohiro Omura said. The thermometers have a margin of error of as much as 20 degrees, so the temperature could have climbed as high as 113.7.

''We think the thermometer may be faulty,'' Mr Omura said. The other two gauges had indicated temperatures were falling.

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