Seismic risk increased at the Fukushima Daiichi nuclear plant following the magnitude 9 earthquake and tsunami that struck Japan on 11 March last year, a new scientific study suggests.
The scientists behind the study are suggesting that authorities should strengthen the security of the Fukushima plant to protect it against large earthquakes it says are likely to directly disturb the region.
The study, ‘Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area’ just published in Solid Earth, an open-access journal of the European Geosciences Union, has used data from more than 6,000 earthquakes. It claims the tremor caused a seismic fault close to the nuclear plant to reactivate.
The scientists make the point that the 11 March 2011 tremor occurred about 160km from the site, and that a much closer one could occur in the future at Fukushima.
Dapeng Zhao, a geophysics professor at Japan’s Tohoku University, and team leader on the study, pointed to how there are a few active faults in the nuclear power plant area, and that the scientists’ 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," said Zhao.
The 11 April 2011 magnitude 7 Iwaki earthquake was the strongest aftershock of the 11 March earthquake. It happened 60km southwest of the Fukushima nuclear power plant, or 200km from the 11 March epicentre.
The Pacific plate is shown in pale yellow
According to the scientists, fluids moving upwards from the subducting Pacific plate to the crust triggered the Iwaki earthquake.
The Pacific plate is apparently moving beneath northeast Japan, resulting in temperature increases and increased pressure of the minerals in it.
The European Geosciences Union said 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," said Ping Tong, lead author of the paper.
After the March 2011 earthquake, the amount of earthquakes in Iwaki increased greatly. For instance, around Iwaki, Japan’s seismic network recorded more than 24,000 tremors from 11 March 2011 to 27 October 2011, up from less than 1,300 detected quakes in the nine years before, according to the study.
The scientists selected 6,000 earthquakes for their study. They were recorded by 132 seismographic stations in Japan from June 2002 to October 2011.
The researchers using a technique called seismic tomography to analyse the data and to take pictures of the Earth’s interior.
"The method is a powerful tool to map out structural anomalies, such as ascending fluids, in the Earth’s crust and upper mantle using seismic waves. It can be compared to a CT or CAT scan, which relies on X-rays to detect tumours or fractures inside the human body," said Zhao.
The scientists said they are not able to predict when an earthquake in Fukushima Daiichi will occur. However, they did infer that ascending fluids observed in the area could mean that such an event is likely to occur in the near future.
They said their results may be useful for reviewing seismic safety in other nuclear facilities in Japan, such as nearby Fukushima Daini, Onagawa to the north of Fukushima, and Tōkai to the south.