A collapse of Mt Fuji need not be caused by major eruption, expert warns

As if Japan wasn't plagued enough by natural calamities this summer, Yukan Fuji (July 23) frets that its namesake, majestic Mt Fuji, might possibly be in danger of a catastrophic collapse. In addition to losing its status as a World Heritage site, the tabloid reports, depending on which side of the mountain were to go -- north, east or west -- it might threaten the existence of 380,000, 400,000 or 150,000 people living in the vicinity.

Fuji, Japan's highest peak at 3,776 meters, is situated atop of the most geologically active parts of Japan, which has been active long before the first humans set foot on the islands. Its current symmetrical form -- referred to as Shin-Fuji or New Fuji -- is believed to have taken shape about 8,000 years ago, making it a mere baby in geological terms.

Shin-Fuji owes its gracious appearance to the fact that not one but two extinct volcanoes repose beneath it: Ko-Fuji or Old Fuji ceased activity about 10,000 years ago after reaching a height of about 3,100 meters. The peak of a much older and smaller volcano, Mt Komitake, which was active about 700,000 years ago, is visible as a bump on Fuji's north slope at an elevation corresponding to the 5th climbing station.

Yukan Fuji asked Professor Yoshiyuki Tatsumi of Kobe University, a specialist in study of magma, what are the chances that the lofty mountain might suffer a partial collapse.

"Based on the pattern of Fuji's past eruptions, the possibility of a future eruption was not estimated until around 2150," Tatsumi said. "But due to the major earthquake that occurred in Tohoku in 2011, magma located deep beneath the volcano may have been activated, which means a spontaneous eruption could happen at any time.

"While volcanoes typically build up pressure to warn of the sense of imminent eruption, we should not ignore the possibility of a collapse of the mountainside," Tatsumi added.

Such collapses can be caused by volcanic eruptions, and also by earthquakes. An example of the former was Mt. Bandai in Fukushima Prefecture, which blew its top in 1888, killing 477 people.

Fuji is believed to have endured a similar eruption around 2,900 years ago. At that time, it regurgitated stones and volcanic cinder estimated to being the equivalent of 10 times the capacity of the Kurobe Dam in Toyama Prefecture.

Should a slope suffer a major collapse, Tatsumi has calculated that detritus from the mountain could flow as far to the east as the cities of Sagamihara and Atsugi in Kanagawa Prefecture.

 "The probability for collapses of this nature to occur are generally calculated at 2% every hundred years," he remarked, adding that the damage to nearby residents would be considerable worse than the ash from an eruption. And there would be practically no time to evacuate.

"The flow in the event of a collapse would be on top of people near the mountain before they realized it. In the fastest case, the destruction could reach Sagamihara in as little as 10 minutes."

Tatsumi said he hopes to develop hazard maps for the area that hypothesize what direction the largest flows would be likely to take, and propose evacuation routes.

Including the earthquake that struck Kagoshima on July 11, there have been three temblors of the intensity of 5 or above around Japan over the past month. They may very well be random; or the phenomena may point to increased seismic activity throughout the archipelago that could mean another major disaster with little or no warning.

© Japan Today