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A sleeping giant stirs Mount Fuji is a cultural icon and Japan’s most important geological feature. Yet until it began rumbling a few years ago, scientists had almost completely ignored it, says David Cyranoski. Is it preparing to erupt again?

ount Fuji, Japan’s tallest peak, last government launched a concerted effort to blew its top on 16 December 1707. study Fuji and learn when and how forcefully MIt spewed for two weeks or more, it could erupt again. Although the initial Digging into the past: with Mount Fuji (top) raining down five centimetres of hot ash on results seem to have produced more ques- showing signs of waking up, scientists led by the capital city Edo — now known as Tokyo tions than answers,Earth scientists who once Toshitsugu Fujii (above) are rushing to gain a — 100 kilometres to the east. In the coun- considered Fuji scientifically uninteresting deeper understanding of the volcano by tryside, ash filled the riverbeds, causing now believe that it holds unexpected clues to examining deposits from previous eruptions. decades of frequent floods. the way volcanoes work. Today, such violence is rarely associated stopping traffic, grounding planes and even with Fuji. “Most people seem to think it is A whole lot of shaking corrupting computer hard drives. But if the completely dead,” says Toshitsugu Fujii, In October 2000, Mount Fuji started experi- eruption came during summer, it could be director of the Volcano Research Center at encing waves of mild earthquakes more than much worse. Each year, 20 million people the University of Tokyo. A sacred mountain 10 kilometres underground. They continued visit the golf resorts, amusement parks and by tradition, it is the legendary home of for about eight months. At their peak in summer homes that dot the volcano’s flanks. numerous deities,and thousands of pilgrims April 2001, more than 100 earthquakes Even more worrying is new evidence that still climb to the summit each year. Even the were recorded in one month, according to Fuji has at times produced deadly pyroclastic less religiously inclined compete for offices the National Research Institute for Earth flows — hot surges of gas, ash and rock that and apartments with a view of the mountain Science and Disaster Prevention (NIED) in can travel at up to 150 kilometres per hour. — visible from downtown Tokyo on clear Tsukuba, vastly more than the average of 15 Depending on the size of the eruption, these winter days.“Everyone thinks it’s pretty,”says per year in the previous two decades. can cover distances from several kilometres Masato Koyama, a geologist at Shizuoka Although the quakes could not be felt at the up to more than a 100 kilometres. University, about 40 kilometres southwest of surface, they carried a worrying message: the In November 2001, the prospect of an the volcano. “They won’t believe it can magma under Fuji is on the move. eruption caused the Japanese government to explode until it does.” If that movement portends an eruption, act.It launched a ¥500-million (US$4.5-mil- But rumblings detected deep beneath the consequences could be dire.“Nowhere in lion), three-year national project to develop Fuji four years ago reminded people that this the world is a large capital city so close to a a basic scientific understanding of Fuji. It giant is only asleep. Concerned that an erup- huge volcano,”says Koyama.At the very least, also allocated a similar amount of money for tion might be in the offing, the Japanese dust and ash would bring Tokyo to a halt by the development of a ‘hazard map’ to detail

volcanic rock known as basalt. Volcanoes of sors, the better that image will be. So Watan- this type usually do not explode violently abe’s team has nearly doubled the number of because, the theory goes, basalt flows freely seismometers on the mountain’s flanks to and does not trap gas inside. about 80, and placed three of them deep in Fuji’s explosive Hoei eruption seems to be the holes bored by Fujii to get a better signal- an exception and so poses a challenge to the to-noise ratio than was previously available. theory, says Yoshiaki Ida, a volcanic geo- Once the arrangement of Fuji’s magma physicist at Himeji Institute of Technology in chambers is understood, the deep low- the western part of Honshu. “Perhaps the frequency (DLF) earthquakes Fuji recently equation of low viscosity with non-explo- experienced may be easier to interpret. siveness is mistaken,”Ida suggests. Scientists hope this understanding will help The Hoei eruption was strange in other them to forecast and plan for eruptions. ways, according to evidence from 300-year- old diaries. To most modern Japanese, the Ready to rumble handwriting from that period is almost inde- Earth movements of various kinds have cipherable,but Koyama,an amateur historian served as warning signs in the past. For as well as a geologist, found historians who instance, in 1991, shallow earthquakes and could make sense of the documents. He has land deformation triggered a mass evacua- pored over some 30 diaries that describe tion around Mount Pinatubo in the Philip- important features of the eruption, which pines shortly before it erupted. Seismic lasted more than two weeks and was inter- activity also set off alarm bells before the rupted by a number of lulls. Mysteriously, the 2000 eruption of Mount Usu on the Japanese intensity of the eruption seems to have picked island of Hokkaido. up in the last few days when it would have been But DLF earthquakes are much less reli- expected to level off,says Koyama.“We need a able. For every example of DLF tremblors the likely threat to lives and property under mechanism concerning the magma that can preceding volcanic activity — several were various eruption scenarios. explain this,”he says. detected before Pinatubo blew, for instance Despite the volcano’s fame, very little — there are many more where nothing hard data on it existed, so scientists had to Emergency planning happens. The result is a great deal of uncer- TY FREE/CORBIS

YAL start virtually from scratch, as if the 3,776- Meanwhile, the Sabo Technical Center, a tainty over how to interpret Fuji’s trembling.

RO metre peak had just been discovered. One semi-governmental institute based in Tokyo, “No convincing model has been pro- reason for the dearth of information may be is drawing up the hazard map. This will show posed,” says Motoo Ukawa, who heads the limited access to the mountain. The treach- in detail the most likely eruption scenarios: research project for predicting volcanic erous snowbound peak is off limits in winter, how much ash, lava and gas would be eruptions at the NIED.And the waveforms of and the red-tape involved in getting samples released; how long the eruption would last; Fuji’s DLF quakes are particularly compli- out of the national park where Fuji is located and what the impact would be on the sur- cated,suggesting that the magma shifts caus- is also forbidding. rounding region. An accurate map depends ing them are complex and hard to model. But the other reason is scientific. “From in part on knowing what kind of emissions The extra seismometers may help, but more a magma perspective I didn’t find it very Fuji is likely to produce, so the Sabo team has time will be needed as the quake frequency interesting,” says Fujii. People who study been examining material from previous has dropped back to previous low levels. magma tend to seek out purer sources, such eruptions on Fuji’s flanks. In the meantime, Fujii and as mid-ocean ridges, where the material What they have found has “The evidence so his colleagues are getting ready comes straight from Earth’s mantle, he surprised geologists. Previously, far suggests that the to release their initial findings explains. Fuji’s magma, on the other hand, evidence of 4,000-year-old pyro- violent eruption in 1707 about the cores in May.Anyoji’s comes through a messy juncture where one clastic flows had been found at does not fit neatly with group will also deliver the haz- tectonic plate bends under another, Fujii two sites around Fuji, but those standard models of ard map around the same time, explains.“It’s the dregs.” were thought to be a fluke of volcanism.” but he says that it will be filled some kind. Basaltic volcanoes with more uncertainty than any Core duties usually do not have pyroclastic flows. In 2001, he has ever worked on.“Where the eruption Despite such reservations, Fujii leapt at the however, Sabo surveyors dug up pyroclastic will take place, what kind of lava will flow, at chance to lead the national Fuji research pro- material at a different site. Initial estimates what speed will it flow — these were all ject, and within weeks he was drilling into dated it to about 1,400 years old.“The find sug- extremely difficult to predict,”he says. the mountainside. The core samples taken gests that pyroclastic flows could be more But if the magma that animates Fuji sounds from these 100- and 600-metre holes on the common than previously thought,” says map like a force of evil, those who admire the northeast slope are now being analysed. They project leader Nobuo Anyoji. mountain’s aesthetics should give credit where should reveal in detail what sort of material The map-makers also need to know credit is due. The 500 cubic kilometres of erupted from the volcano in 1707, which in where the magma is, how it is moving and magma that Fuji has deposited on its flanks turn should help scientists to understand what forces act on it. But the nature of Fuji’s over the past 100,000 years has kept the moun- what caused that eruption. “To know what’s magma chambers is proving elusive. In an tain beautifully symmetrical in the face of con- going on today, we need to know what has effort to pin them down, Hidefumi Watan- stant erosion. Nevertheless, below the surface happened in the past,”Fujii says. abe,Fujii’s colleague at the Volcano Research disaster lurks. “If the magma moves, sections The evidence so far suggests that the 1707 Center, is leading the effort to monitor the would begin to collapse,”says Richard Arculus, eruption, known as Hoei for the period in volcano’s seismic movements. Shock waves a geochemist at the Australian National Uni- which it occurred, does not fit neatly with from quakes work almost like an under- versity in Canberra.“It may be culturally sad, standard models of volcanism.Records show ground sonar to produce an image of the but that’s the fate of all volcanoes.” ■ that it was a particularly violent event. But size, position and movement of magma David Cyranoski is Nature’s Asian-Pacific correspondent. Fuji consists almost entirely of a type of chambers. The greater the number of sen- ➧ http://hakone.eri.u-tokyo.ac.jp/vrc/VRC.html