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What did these devices show before and during the eruption?
The tiltmeter and GPS data showed no change in the run-up to or during the eruption. “In big eruptions, magma rises, and the tilmeters and GPS would pick it up,” says Toshikazu Tanada, who heads volcano research at Japan's National Research Institute for Earth Science and Disaster Prevention in Tsukuba. “But there was no sign of this.” Even though there are relatively few instruments at Ontake, Tanada adds, it is unlikely that they would have missed significant magmatic activity.
The seismometers did show a spike in activity. According to figures from the Japan Meteorological Agency, after a quiet August with no seismic activity the number jumped to 52 on 10 September and 85 on 11 September. After that, the rate settled at a (still-high) 10–20 per day. Also, 11 minutes before the eruption, the seismometers detected a volcanic tremor, a type of seismic activity distinct from earthquakes that often accompanies eruptions.
Why did the government not issue a warning and evacuate people?
Increased seismic activity at a volcano, like that observed in early September, can signal a greater chance of an eruption, but often it does not. In 2011, the same type of increase was observed at Ontake without any subsequent eruption. Unlike some volcanoes, such as Mount Usu in Hokkaido, Ontake does not have an easily predictable cycle. “It's a very quiet mountain,” says Tanada. “Each active volcano has its own characteristics.”
Most volcanoes show erratic seismic activity. Mount Fuji, for example, often has dozens of small earthquakes in a day, and others can have more than 100. If the rules were to restrict people from a volcano when it showed the activity seen at Ontake before last week, the authorities would have to close more than 10% of Japan’s volcanoes to visitors. “We could just restrict everywhere, but people don’t want that,” Tanada says.
Access is now restricted to within 4 kilometres of the crater. Two other volcanoes in Japan are currently subject to similar restrictions, and at five others, hikers are barred from approaching the craters.