Japan's crippled nuclear plant at Fukushima needs to beef up safety measures to face the risk of a large earthquake, scientists said on February 15. The alarm was sounded by a trio of seismologists in Japan and China, who say the risk is an indirect cause of the March 11 tsunami-generating super-quake.
"The security of the nuclear plant site should be strengthened to withstand potential large earthquakes in the future," the team said in a study published in a specialist journal.
The concern stems from a computer model of the crust and mantle lying under north-eastern Japan. Using a technique called seismic tomography, the team analysed data from a vast network of sensors which record waves of energy propelled through the ground by earth tremors. By noting the type of wave and the time it takes for the wave to travel between sensors, scientists can build a picture of the layers of sub-surface rock, rather like a CAT scan in medicine shows a cross-section of the body. The study looked at the aftermath of a magnitude 7.0 quake that occurred on April 11 6.4 kilometres (4.06 miles) beneath Iwaki, 60 kilometres (37.5 miles) south-west of the Fukushima Daiichi plant.
It was one of the heftiest aftershocks of the 9.0-magnitude March 11 quake and the biggest that was recorded on land. The picture points to a low-activity fault that was jolted furiously into life by the big quake. From March 11 to October 27 last year, the carpet of sensors around Iwaki recorded an astonishing 24,108 shocks measuring at least 1.5 magnitude, 23 of them notching up a powerful 5.0 or more.
The paper appears in Solid Earth, a peer-reviewed open-access journal published by the European Geosciences Union (EGU). The Fukushima Daiichi plant was swamped by towering waves that paralysed cooling systems and sent three reactors into meltdown. Radioactive materials leaked into the air, soil and the sea in what was the world's worst nuclear accident since Chernobyl a quarter-century earlier.