So should Japan be on red alert? “We cannot establish a direct relation of cause and effect between quakes and volcanic eruptions, even if statistically the former lead to an increase in the latter,” Brenguier says. “All we can say is that Mount Fuji is now in a state of pressure, which means it displays a high potential for eruption. The risk is clearly higher.”
Science, however, has no way of predicting when this might happen. But there is a precedent. The last eruption of Mount Fuji occurred in 1707. It projected almost a billion cubic metres of ash and debris into the atmosphere, some of which reached Tokyo (then called Edo) 100km away. It was preceded, 49 days earlier, by a magnitude 8.7 quake to the south of Japan that, in conjunction with the tidal wave it raised, claimed more than 5,000 lives. This time, more than three years have already passed since the Tohoku quake. But that does not mean that Mount Fuji, under the constant supervision of Japanese geologists, is slumbering.
Come what may, the method developed by the Franco-Japanese team for investigating volcanic areas should improve the accuracy of efforts all over the world to assess the risk of major volcanic eruptions.