Scientists have discovered signs of a possible imminent eruption of Japan's most famous volcano, Mount Fuji on Honshu. At least this is what many headlines in the press read. According to an article of the Japanese news agency Kyodo the pressure in the magma chamber beneath the volcano Mt Fuji has drastically increased after the tsunami in March 2011 and a magnitude 6.4 quake near the volcano four days later, and is currently higher than it was before the last eruption in 1707 some 300 years ago. Scientists at the National Research Institute for Earth Science and Disaster Prevention have calculated that the tectonic shifts in the past year caused an increase in pressure to 1.6 megapascals. That is sixteen times as much as the threshold of 0.1 megapascals needed for an eruption, they said. Such statements should be viewed with caution, because the figures are obtained by indirect calculations, which may be subject to very large errors. Secondly, pressure data for magma chambers inside a volcano without an associated depth profile have little significance. Pressure inside a volcano, as everywhere, increases naturally with depth. What matters is the difference between the lithostatic pressure (weight of the volcanic edifice) and the gas pressure of the gases in the magma. If this difference becomes greater than the tensile strength of the rock, it can open cracks and let the magna erupt. The scientists admit that these calculations alone don't necessarily mean that an eruption is imminent and other parameters such as seismicity and deformation currently don't show signs of an impending eruption.

After the earthquake in March 2011, a professor from Ryukyu University warned in May that a larger eruption of Fuji should be expected within 3 years, based on the observations that:
- Steam and gases emissions from the crater have increased.
- Water eruptions were observed in the area.
- Huge pit craters had appeared on the flank of the mountain, from which gases escape.
- Above all, a 34 km long fault was discoverd recently, which extends right under the volcano.

This fault could even cause the collapse of the whole mountain if there is a large-enough movement along the fault. Such a collapse would cause devastating pyroclastic flows, debris avalanches and extensive lahars, and be a disaster of unprecedented scale. The latest measurements show surely that the huge tectonic movements of 2011 have increased pressure on the magma chamber, but additional parameters such as deformation of the volcano and earthquakes are needed to make a reliable forecast for a new eruption. Worrying in this context, however, is that the last eruption in 1707 was most likely influenced, if not triggered by an earthquake earlier."It's possible for Mount Fuji to erupt even several years after the March 2011 earthquake, therefore we need to be careful about the development," a spokesman for the National Research Institute for Earth Science and Disaster Prevention said. A report from 2004 estimated the probably damage of a new eruption to around 2 billion USD. Recent studies assume an increased risk of earthquakes in the Tokai, Tonankai and Nankai regions, which lie at the edge of the Nankai Trough, where a larger quake is expected. In the worst case, a series of simultaneous severe quakes could cause up to 323,000 deaths and additionally trigger an eruption of Mount Fuji, the gloomy conclusion of the latest report reads.


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