Japan's volcanic eruptions linked to quakes
TOKYO — About 250 climbers had been enjoying the autumn foliage near the summit of Mount Ontake in splendid weather when huge, black plumes of smoke suddenly rose from the mountaintop at 11:52 a.m. on Sept. 27.
Rocks showered down from the sky, leaving a layer of volcanic ash everywhere. Many climbers who barely escaped with their lives said they never imagined Mount Ontake would erupt.
The death toll from the eruption has topped 50, making it the nation's worst volcanic disaster in the postwar period in terms of casualties. It was a hydrovolcanic explosion, in which a blast of steam is generated when groundwater is heated to extremely high temperatures by magma.
Japan has 110 active volcanoes, or 7 percent of those in the world. At the same time, Japan is prone to earthquakes. This is no coincidence.
The nation has two volcanic fronts, where a band of volcanoes are concentrated. One spans from the Chishima islands to the islands of Izu and Ogasawara, while the other is an axis stretching from the Chugoku region to the Satsunan islands. Each of the fronts runs parallel to an oceanic trench in the Pacific Ocean.
In an oceanic trench, an oceanic plate subducts beneath a crustal plate. Movement of the oceanic plate causes giant earthquakes near the boundary with the crustal plate. The oceanic plate also draws water into the mantle. Rocks melt when the water reaches extremely high temperatures and high pressure, generating magma.
The upward movement of magma causes the eruptions that create volcanoes. "In Japan, there has been increased seismic activity, which leads to increased volcanic activity," said Yoshinobu Tsuji, a special visiting research fellow at the Building Research Institute.
In recent decades, the nation has been hit by such giant temblors as the 1995 Great Hanshin Earthquake and the 2011 Great East Japan Earthquake. Especially since the 2011 disaster, volcanic earthquakes that could be warning signs of a possible eruption have been observed beneath 21 volcanoes from Hokkaido to Kyushu. At Mount Fuji, which has been dormant for more than 300 years, there has recently been underground seismic activity.
After the Great East Japan Earthquake, Mount Ontake, which is located a short distance from a volcanic front, remained dormant. Other nearby volcanoes such as Mount Norikura and Mount Yaki, both of which straddle the border between Gifu and Nagano prefectures, experienced a series of earthquakes of magnitude 3 or 4.
Masaaki Churei, a former official at the Japan Meteorological Agency who is familiar with the links between earthquakes and volcanoes, said: "When a huge earthquake occurs off the Sanriku coastline, volcanoes that have remained dormant tend to erupt two or three years after the earthquake. It's possible the Great East Japan Earthquake instigated the eruption of Mount Ontake."
"Over the past century, volcanoes in Japan have been relatively dormant compared to the 17th to 19th centuries," said Toshitsugu Fujii, a professor emeritus at the University of Tokyo and the head of the Coordinating Committee for Prediction for Volcanic Eruptions. "However, it shouldn't be a surprise whenever and wherever a volcano erupts. It's Japan's fate to have volcanic eruptions."