Mount Ontake (Ontakesan) is the second tallest volcano in Japan, topping out at 3,067 m. It is one of Japan’s most sacred mountains and has been such for over 1,000 years. As such, there is a massive amount of human traffic on the mountain, ranging from pilgrims to skiers to simple hikers. The number of non-religious hikers has increased significantly in recent years.
Pilgrims practice a blend of Shinto, Buddhism and ancient shamanic rites. They come from across Japan to walk the Ontake Pilgrimage Trail to the summit. For those who do not want to walk the entire way, there is a ropeway that takes visitors within a couple hours hike of the summit. There are numerous shrines on the volcano, sacred ponds, and lodges that visitors can stay in during the pilgrimage season.
A lower trail starts in Otaki Village. It has a pair of sacred waterfalls that pilgrims purify themselves under before starting up the mountain. Most pilgrimages take place during spring through fall.
In the winter, Ontake has two active smallish ski areas, Ontake 2240 and the Kaida Kogen Maia Ski Area. The Ontake 2240 ski resort normally opens in early Dec. Its opening was delayed for two months following the Sept 27, 2014 eruption and subsequent establishment of a new exclusion zone around the site of the eruption. The resort normally stays open until early May each year.
Volcano Cams in Japan has live feed from several webcams for Japanese volcanoes. Volcano Discovery has a link to the same Ontake webcam also. Reported ski resort webcams were either nonexistent or not active at this writing.
Sept 27, 2014 Eruption
The most recent phreatic eruption of Mount Ontake was the worst volcanic disaster in Japan since 1926. This one took 63 lives. It took Japanese Self Defense forces months to recover all bodies from the eruption. The timing of the eruption near midday contributed to the high death toll since many hikers were near the summit.
Precursors to the eruption were detected with seismometers and tiltmeters about 10 minutes before the eruption but no warning was issued before the eruption began.
Early analysis of the eruption by the Japanese Meteorological Agency (JMA) determined it was phreatic. There was concern that it could evolve into a magmatic one based on experience with similar Japanese volcanoes.
Perhaps the best post-eruption analysis of the events was the 2016 Special issue “The phreatic eruption of Mt. Ontake volcano in 2014” by Yamaoka, et al. I will be borrowing liberally from this document in the paragraphs to follow.
One of the positive (if there can be a positive) outcomes from the large number of people on the volcano when it erupted is unusually detailed observations, personal interviews, photo, video and other records of a phreatic eruption, something that is rarely captured this well.
At a top level, the eruption opened three major craters at the head of Jigukudani valley. The center crater was the main vent. A pyroclastic flow traveled 2.5 km down the valley from the central vent. Ballistic ejecta started tens of seconds after the eruption began, reaching up to 950 m from the vent.
The eruption proceeded in three phases: pyroclastic flow, pyroclastic fall with muddy rain, and muddy water overflowing from the crater. The pyroclastic flows began without any surface warning. They were generally 30 – 100 C at the summit. During the period the pyroclastic flow was active, lapilli and volcanic block ballistic ejecta fell within a kilometer of the crater. Eruption tephras were ejected as pyroclastic flows, ballistics and airfall. Total discharge mass of the 2014 eruption is about half that of the 1979 eruption.
The tephra phase was followed by a lahar generated by water directly ejected from the craters. It flowed 5 km down the valley to the south of the craters.
Seismic analysis of pre-eruption earthquakes found a 10-minute period immediately before the eruption where volcano-tectonic earthquakes migrated upward and extended in the N-S direction, roughly the orientation of the newly opened vents. This is interpreted as the tensile opening of a crack 0.3 – 1.0 km below the surface allowing hydrothermal fluid to flow through the crack to the surface. This conclusion is supported by changes recorded by two tiltmeters in the 10 minutes before the eruption.
The source of the eruption was very shallow, less than 2 km and likely less than 1 km. Volcanic fluids and tephras came from a shallow hydrothermal system under the summit. There is some disagreement about the depth of this system that needs additional data to get a better handle on. A larger hydrothermal system may exist beneath the crater and the eastern flank. There has been uplift and subsidence measured on the eastern flank between 2007 and 2014.
There are two possible causes for this activity. One would be a magmatic intrusion before 1979 that started the sequence of phreatic eruptions (1979, 1991, 2007 and 2014). The intrusion took place around 5 km. Although no new magma has been erupted yet, the possibility exists that a phreatic eruption can evolve into a phreatomagmatic one. The second possible cause is serial pressurizing and breaching of the shallow hydrothermal system.
The volcano itself sits at the southern end of the Northern Japan Alps. It occupies a 4 x 5 km caldera created by four large stratovolcanoes active 680 – 420,000 years ago. Ontake was quiet for over 300,000 years before recent activity began.
The VOGRIPA database lists seven major eruptions from Ontake starting with a VEI 6.7 95,700 years ago that ejected some 50 km3 bulk volume. As an aside, I would guess that eruption was the caldera forming eruption, though I have not yet found confirmation in the literature, making this pure speculation.
The remaining six eruptions range between VEI 5.0 – 6.0, mostly ejecting 1 km3 bulk volume with the largest eruption ejecting 10 km3 in 94,900 years ago. The string of large eruptions ended some 70,000 years ago. There is a valley-filling debris avalanche called the Kaida at the ESE foot of Ontake dated around 50,000 years ago. It is greater than 0.3 km3 in volume.
The volcano is largely andesitic with basalts, dacites and rhyolites present. Its four closest neighbors to the north are from south to north – Norikura, Yakedaka, Washiba – Kumonotaria, and Midagahara. All lie within 90 km and are along a generally N – S line. The first two and last listed are stratovolcanoes. Washiba is a shield volcano. Haku-san lies some 65 km to the WNW and Kita Yatsuga-take lies a similar distance to the ENE. All 6 of these volcanoes are considered dormant.
There have been at least four magmatic eruptions from Ontake in the last 10,000 years. The most recent of these was from Gonoike crater around 6,000 years ago. Phreatic eruptions appear to take place every several hundred years. Active fumaroles in a valley SW of the summit have been reported for several hundred years.
The first recorded eruption historic eruption from Ontake took place in Oct 1979. This was a VEI 2 that discharged new material. It opened new craters near the top of Jigukudani valley. The eruption began with vapor emission early in the morning on Oct 28. The plume changed to ash three hours later and became more vigorous through 1100. Tephra was continuously ejected for the rest of the day. The following day was vapor only. The eruption opened a fissure 300 m south of the summit. There were no fumaroles in the area before the eruption. The eruption plume was as high as 1.5 km above the volcano. Vapor emissions and earthquakes declined in the months following the eruption.
A M 6.8 earthquake 12 km SE of the summit took place on Sept 14, 1984. This triggered a landslide on the south flank of the volcano that killed 29. This did not change ongoing steam emissions from the 1979 craters.
There was a small phreatic eruption from one of the new vents created in 1978 in 1991. Earthquake swarms started in late April and continued through May. By mid-June, 170 earthquakes and 8 tremor episodes were recorded. There were also white steam plumes to 200 m above the crater. By July, seismicity had decreased. The summit still weakly emitted steam, though only to 100 m.
Seismic activity took place with varying degrees of intensity between 1991 through the 2007 eruption, often receding to background levels.
Activity began to increase along with minor inflation in Dec 2006. The earthquakes were shallow directly below the summit. By Jan tremor had returned and was accompanied by inflation. Fumaroles increase in mid-March. Occasional fumes at the summit were detected by surveillance camera. At this point, JMA decided that a magmatic intrusion beneath the volcano was within 4 km of the summit. Fresh volcanic ash was observed from one of the 1979 craters at the end of May. The ash indicated the eruption included some juvenile magma release to the surface, though the actual date was unknown.
The period since 1979 represents Ontake’s most active during the past 250 years. This was accompanied by a very long period earthquake and crustal deformation.
JMA monitored fumarole height and temperature data from 1979 to present. It did not take data 1981 – 1988. One oddity noted was relatively low fumarole activity the three years before the 2007 and 2014 eruptions. Seismic activity was also monitored. It increased before the 1978 and 2007 eruptions. It was relatively low in the 3 years before 2014.
JMA detected unusual seismic activity Sept 10 – 11, 2014, two weeks before the eruption. Daily counts were the largest since the 2007 eruption, though no tremor or deformation was detected. There were low frequency earthquakes detected but were much fewer in number than those associated with the 2007 eruption.
Tectonics of Honshu Japan are driven by the subduction of the Pacific Plate beneath the Eurasian Plate to the north and the Philippine Sea Plate beneath the Eurasian Plate to the south. The line between the two plates is roughly to the east of Tokyo. The offshore subduction line in the north part of Honshu is the Japan Trench. To the south, it is the Nankai Trough. The main phase of subduction of the Philippine Sea Plate began some 8 Ma.
Comparison of crustal strain rates suggest that much of the deformation occurring in central Japan is not associated with subduction. Rather it is associated with the collision of SW Japan with NE Honshu. And the mountain building associated with that collision rather than subduction
The Yamaoka et al summary of the eruption discusses ways to mitigate future disasters. The problem they have is much of the long-term data from precursors while present are ambiguous at best and not useful for real time forecasting as they operate on the order of years rather than days. They are currently useful for post event analysis, though that may change as we get better at interpreting the data. There are some midterm changes in seismic and deformation that might be useful, with an improved local geodetic network or InSAR data analysis.
As we have seen from White Island, the more people you get on an active volcano, even one that is not regularly active like Ontake, the more cumulative risk you take for a deadly event. And this one Appears to be waking up a bit since 1979.