The Phreatic Eruption of Mt. Ontake Volcano in 2014

Editorial Board (December 2016)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

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Masato Furuya, Hokkaido University, Japan
Solid Earth Science

Nozomu Nishitani, Nagoya University, Japan
Space Science

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Earth, Planets and Space
Yamaoka et al. Earth, Planets and Space (2016) 68:175
DOI 10.1186/s40623-016-0548-4

Open Access

PREFACESpecial issue “The phreatic eruptionof Mt. Ontake volcano in 2014”
Koshun Yamaoka1*, Nobuo Geshi2, Takeshi Hashimoto3, S. E. Ingebritsen4 and Teruki Oikawa2
Introduction
Mt. Ontake volcano erupted at 11:52 on September 27,
2014, claiming the lives of at least 58 hikers. This eruption
was the worst volcanic disaster in Japan since the 1926
phreatic eruption of Mt. Tokachidake claimed 144 lives(Table 1). The timing of the eruption contributed greatly
to the heavy death toll: near midday, when many hikers
were near the summit, and during a weekend of clear
weather conditions following several rainy weekends. The
importance of this timing is reflected by the fact that asomewhat larger eruption of Mt. Ontake in 1979 resulted
in injuries but no deaths. In 2014, immediate precursors
were detected with seismometers and tiltmeters about
10 min before the eruption, but the eruption started
before a warning was issued.
The Coordinating Committee for Prediction of Volcanic Eruption met the next day to investigate observations by institutions such as the Japan Meteorological
Agency (JMA). They concluded that the eruption was
phreatic, based on analysis of volcanic ash collected
immediately after the eruption, and also concluded thatthe eruption could evolve to a magmatic one, based on
experience at other volcanoes. Urgent research began
under the Grants-in-Aid for Scientific Research program
funded by the Japan Society for the Promotion of Science (JSPS). This special issue was planned to collect the
research carried out in response to the 2014 eruption of
Mt. Ontake.
Before reviewing the papers in this issue, we summarize the recent activity of Mt. Ontake and selected resultsfrom JMA monitoring. Magmatic activity is relatively low
in comparison with other active volcanoes in Japan. Geological studies show that at least four magmatic eruptions
occurred over the past 10,000 years, the most recent from
Gonoike crater around 6000 years ago, and that phreatic

eruptions occurred every several hundred years (Oikawa
et al. 2015). No historical records of eruptions prior to1979 have been found, although fumarolic activity in the
Jigukudani valley to the southwest of the summit (Fig. 1)
has been reported for the past several hundred years(Oikawa 2008).
On October 28, 1979, the first recorded eruption inhuman history occurred, a phreatic eruption with total
discharged mass of about 200,000 tons corresponding to
VEI 2. This eruption opened new craters near the top of
Jigukudani valley (Fig. 1). In 1991, a very small phreatic
eruption occurred from one of the vents created by the
1979 eruption. In 2007, another small eruption occurredfrom the same vent, and a very long period (VLP) earthquake and crustal deformation were detected (Nakamichi
et al. 2009). No further eruptions occurred until September 2014.
Figure 2 shows results of long-term monitoring by the
JMA. Fumarole height data (Fig. 2a) are lacking from1981 to 1988 but show a relatively low level of activity during 3-year periods before both the 2007 and 2014
eruptions. Fumarole vent temperatures, measured intermittently, decayed through 2012, although there are few
more recent measurements (Fig. 2b). Seismic activity
near the summit has been monitored from 1988 to present and increased in association with the eruptions in1991 and 2007 (Fig. 2c). The level of seismic activity was
low during the 3-year period before September 2014.
JMA detected unusual seismic activity on September
10–11, 2014, two weeks before the eruption. Daily events
counts were the largest since the 2007 eruption, but novolcanic tremor or crustal deformation was detected.
Low-frequency earthquakes were detected, but weremuch fewer in number than those observed in association with the 2007 eruption. This is the main reason that
the JMA chose not to raise the volcano warning level, andmakes comparison between the 2007 and 2014 eruptions
a primary issue in many of these studies.

*Correspondence: kyamaoka@seis.nagoya‑u.ac.jp
1
Graduate School of Environmental Studies, Nagoya University, Nagoya,Japan
Full list of author information is available at the end of the article© The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

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The Phreatic Eruption of Mt. Ontake Volcano in 2014
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Table 1 List of recent Japanese eruptions that claimed >10 lives (supplemented to Japan Meteorological Agency 2013)
Date (Y/M/D)

VolcanoVictims

Description

1721/6/22

Asama

15

Ballistic ejecta

1741/8/18

Oshima‑Oshima

1467

Tsunami1779/11/8‑9

Sakurajima>150

Ballistic ejecta and lava flow

1781/4/11

Sakurajima

15

Tsunami caused by an off‑shore eruption

1783/8/5

Asama

1151

Pyroclastic flow, mud flow and flood by a collapse of natural dam

1785/4/18

Aogashima

130–140

Phreatomagmatic eruption?

1792/5/21

Unzen>15,000

Debris avalanche and tsunami caused by it

1822/3/12

Usu

50

Pyroclastic flow

1856/9/25

Hokkaido‑Koma

21–29

Ballistic ejecta and pumice flow

1888/7/15

Bandai461

Debris avalanche

1900/7/17

Adatara73

Destruction of a sulfur mine

1902/8/7

Izu‑Torishima

125

Phreatomagmatic eruption

1914/1/12

Sakurajima

58–59

Ballistic ejecta and lava flow

1926/5/24

Tokachidake

144

Mud flow

1940/7/12

Miyake11

Ballistic ejecta

1952/9/24

Bayonnaise

31

Phreatomagmatic explosion with submarine eruption

1958/6/24

Aso31

Ballistic ejecta

1991/6/3

Unzen

43

Pyroclastic flow

2014/9/27

Ontake58 (+5 missing)

Ballistic ejecta

Fig. 1 Location map of Mt. Ontake volcano, modified from Fig. 1 of Oikawa et al. (2016)2

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Height above summit

a Fumarole Height

No DataYear

Temperatureb Fumarole Temperature

No Data

Year

Monthly numberc Monthly Number of Earthquakes

No DataYear
Fig. 2 Results of long‑term monitoring of volcanic activity by the Japan Meteorological Agency: a fumarole height based on visual measurements;
b fumarole temperature measured at an active vent of the 1979 eruption; c monthly number of earthquakes near the summit of Mt. Ontake. Earth‑quakes are counted for the same station under a constant criterion using S‑P time and maximum amplitude in seismogramsContents of the special issue
In this section, we briefly review the papers of the special issue together with related papers published in otherjournals. The papers are classified into five categories
depending on the principal study methodology.

with relatively low destructive force travelled approximately 2.5 km downvalley from this central vent. Ballistics ejecta emerged tens of seconds after the beginning of
the eruption, reached distances of up to 950 m from thevent, and attained maximum initial velocities estimatedat 111 m/s.
Oikawa et al. (2016) describe the sequence of the eruption based on interviews and visual records such as still
photographs and videos taken by hikers, which provide
precise time records of the eruption. The interviewees included mountain guides and workers in mountainhuts. Oikawa et al. (2016) divide the eruption into threephases: PDC flow, pyroclastic fall with muddy rain, andmuddy water overflowing from the crater. The eruption
began with dry pyroclastic density currents and without
any precursory surface phenomena. The temperature of
the PDCs in the summit area was generally 30–100 °C.
During the period over which the PDCs covered the surface, many lapilli and volcanic block-sized ballistic ejecta
fell within 1 km of the crater.

Visual observations

The 2014 eruption of Mt. Ontake provided an unusually rich set of visual records of phreatic eruption. Hikers near the crater took still photographs and videos
from many locations immediately after the onset of theeruption. Researchers took photographs and videos fromnews-media helicopters in good weather conditions.
These visual data are valuable for process studies of phreatic eruptions, which have rarely if ever been recorded insuch detail. Two papers (Kaneko et al. 2016; Oikawa et al.
2016) are based mainly on the visual data; see also Maeno
et al. (2016) for eruption chronology based on combined
analysis of visual images and the tephra sequence.
Kaneko et al. (2016) describe the spatial characteristicsof the eruption based in large part on aerial observations
made the following day. Three major craters opened at
the head of Jigukudani valley (Fig. 1); the central crater
was the main vent. A pyroclastic density current (PDC)

Geological observations

During the 2014 Mt. Ontake eruption tephra was ejected
as PDC, ballistics, and air fall. Takarada et al. (2016)

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Kato et al. (2015) perform a high-resolution analysisof seismicity associated with the 2014 eruption using amatched-filter detection technique. They succeed in relocating the volcano-tectonic earthquakes (VT) and longperiod earthquakes (LP) from August and September
2014. VT seismic activity increased from 6 September
to 11 September, and a change in the frequency-magnitude distribution of the seismic activity (b-value) was also
observed. During the 10-minute period before the eruption, VT earthquakes migrated upward and extended in a
N–S direction, which roughly coincides with the orientation of the newly opened vents. The authors emphasize that
hypocenter migration immediately before the eruption may
reflect the movement of hydrothermal fluids to the surface.
Zhang and Wen (2015) (Geophysical Research Letters)
apply the match-and-locate method to detect and precisely locate seismic activity associated with the 2007 and2014 eruptions. The results for 2007 show less variation in
the epicenter location than those associated with the 2014
eruption, consistent with the results of Kato et al. (2015).
Maeda et al. (2015) analyze a very long period earthquake (VLP event) that occurred 25 s before the startof the eruption and infer opening of an ENE-WSW-oriented tensile crack at a depth of 0.3–1 km from the surface. They infer that hydrothermal fluid flowed throughthe crack toward the surface.
Ogiso et al. (2015) use the spatial amplitude distribution of tremor data associated with the eruption to track
the source location of the tremor. They find that it was
located beneath the crater and descended 2 km over thecourse of several minutes following the onset of the eruption; this appears to be a robust solution.
Terakawa et al. (2016) (Nature Communications) analyze temporal change in the focal mechanism of earthquakes beneath the craters to document the stressvariation associated with the phreatic eruption. Substantial deviation from the average stress field is observed in
the focal mechanism in the period before the phreatic
eruption.

and Maeno et al. (2016) document the total volumeof tephra and the size of the eruption and Sasaki et al.
(2016) describe the resulting lahar. Kaneko et al. (2016)
and Tsunematsu et al. (2016) analyze the distribution ofballistic ejecta and Tsunematsu et al. (2016) simulate ballistic trajectory and calculate a landing energy of 104 J,
essential data for future shelter designs.
Takarada et al. (2016) estimate discharged mass of the
fallout deposit by a segment-integration method and finda total mass of around 1 × 106 tons, more than 90% of
which occurs proximal to the craters. The discharged
mass of the 2014 eruption is about half that of the 1979
eruption.
Sasaki et al. (2016) describe the syneruptive-spouted
type lahar generated by water directly emitted from thecraters. The lahar flowed downvalley south of the crater
to a distance of 5 km. A rough estimate of the emplaced
volume is 1.2 × 105 m3, about 1/10 of the total discharge
volume. The chemical characteristics of the lahar material are the same as those of the tephra, indicating a common origin.
Maeno et al. (2016) reconstruct the eruption sequencebased on observations of PDC and fallout deposits in theproximal area near the crater. The deposits indicate thatthe eruption began dry but evolved to wet. The deposits show, from bottom to the top, gravity-driven dilute
PDCs from the vent-opening phase; fallout from a vigorous moist plume during vent development; and wet ash
fall. The particle concentration and initial flow velocityof the PDCs are estimated to be 2 × 10−4–2 × 10−3 and
24–56 m/s, respectively.
Tsunematsu et al. (2016) estimate ejection velocities
of ballistic projectiles by comparing simulation resultswith the observations by Kaneko et al. (2016). To accountfor the observed distribution in the summit area, ballistic projectiles are ejected at a vertical angle of 20° fromvertical and at an azimuthal angle 20° clockwise from thenorth. The optimal ejection velocity is 145–185 m/s fora particle diameter of 20 cm with a drag coefficient of
0.62–1.01, and the mean landing energy is of the orderof 104 J.

Geodetic observations

Geodetic observation reveals deformation processes andcan permit direct estimation of the location and geometry of the source of deformation. The four papers dealing with geodetic observations all consider both the 2007
and 2014 eruptions.
Takagi and Onizawa (2016) use Global Navigation Satellite System (GNSS) observations and model the inflation that occurred before the 2007 eruption as a volume
increase of 6 × 106 m3, mainly as crack opening (diking)at an estimated depth of 5 km below sea level. A smaller
volume change of 0.38 × 106 m3 at a depth of about 1 km
beneath the surface is estimated for the period before

Seismological observations

Seismological observations are used to analyze source
processes of earthquakes and tremors and locate theirsources. Kato et al. (2015) and Zhang and Wen (2015)
provide detailed descriptions of the hypocenters associated with the eruption. Maeda et al. (2015) infer crack
opening immediately before the eruption. Ogiso et al.
(2015) track the source location of tremor beneaththe crater. Finally, Terakawa et al. (2016) reveal stresschanges associated with the eruption, strongly implying
fluid-pressure changes.

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well-known mineralogy of porphyry copper systems. The
ash contains partly altered volcanic rock fragments andvolcanic glass accompanied by alteration minerals, as is
typical of non-juvenile hydrothermal-eruption deposits. The mineral composition indicates that the ash wasderived from depths of less than 2 km.
Mori et al. (2016) describe a series of measurements of
volcanic gas near the 2014 eruption vents which revealthat the volcanic gas was not directly emitted frommagma but was influenced by hydrothermal system.
The SO2 flux was more than 2000 t/day one day after the
onset of the eruption but decreased to 130 t/day over thecourse of the following two months. Taking the significant decline in the SO2/H2S ratio into account, it is likelythat the H2S flux remained about 700–800 t/day for twomonths after the onset of the eruption. Such a sustained
high flux of H2S is a peculiar feature of Mt. Ontake relative to other major volcanoes. A similar pattern of variation was observed following the 1979 eruption.
Sano et al. (2015) (Nature Communications) describe
the results of intermittent monitoring (13 samples/33 years) of helium-isotope data from 1981 to present in the vicinity of Mt. Ontake. These data show anincreasing 3He/4He ratio from 2000 to 2014 at the sitenearest to the crater. This suggests increasing contribution of mantle helium and may reflect magmatic activity
beneath Mt. Ontake volcano.
Sabry and Mogi (2016) mapped the electrical resistivity structure beneath Mt. Ontake using a GREATEM(GRounded Electrical source Airborne Transient ElectroMagnetic) system. A low-resistivity layer beneath theeastern flank of the volcano suggests the presence of ahydrothermal zone at around 500 m depth, potentiallyconnected to the phreatic eruption.

the 2014 eruption. The apparent lack of deeper volume
change prior to the 2014 eruption is viewed as consistent
with the water-level observations of Koizumi et al. (2016).
Koizumi et al. (2016) analyze a groundwater-pressure
monitoring record maintained from 1998 on at a borehole 10 km southeast of the summit. That record showsa clear drop in pressure prior to the 2007 eruption but nopressure change before or after the 2014 eruption, consistent with the inference of little or no additional magmatic intrusion.
Miyaoka and Takagi (2016) apply a stacking method toincrease the signal-to-noise ratio of GNSS data and compare the period before the 2014 eruption to that before
the 2007 eruption. The 2014 data reveal minor crustal deformation caused by volume change in a shallow
source beginning one month before the eruption. The
volume change is unresolvable but appears similar to thatassociated with the 2007 eruption.
Murase et al. (2016) use leveling-survey and GNSS
data from 2002-present to estimate a source model ofcrustal deformation. Data from 2006 to 2007 are used toconstruct a model of crack opening for the 2007 eruption. From 2007 to 2013, no GNSS change was observed,
but leveling surveys showed uplift on the eastern flank,suggesting continued crack opening there. During the2014 eruption, baseline shortening and subsidence wasobserved, indicating deflation of the source beneath thesummit.
Observations and analysis of the hydrothermal system

Geochemical study of volcanic fluids and tephra provides
detailed information regarding the hydrothermal systemthat appears to have fed the eruption. Five papers show
the possible range of depth and temperature of the source
region as well as process that may connect the hydrothermal system with underlying magma. Based on the lack ofjuvenile materials, sulfur isotope equilibrium temperatures of 270–280 °C, and some indications of magmatic
influence, these eruptions may be classified as “hydrothermal eruptions,” in which most of eruption energy isderived from the hydrothermal system itself, indirectly
influenced by deeper magmatic intrusion.
Ikehata and Maruoka (2016) analyze pyroclastic material from the 2014 eruption and show that it derives
from acidic alteration zones. Sulfur isotope systematics
show that the pyroclastic material equilibrated at a temperature of 270–280 °C. Pyroclastic material from thesomewhat larger 1979 eruption is very similar in termsof mineral assemblage and sulfur isotope composition,with the exception of the sulfur isotope composition ofanhydrite.
Minami et al. (2016) compare the mineral assemblageof the volcanic ash from the 2014 eruption with the

Discussion
Shallow source structure

Several lines of evidence indicate that the source of thephreatic eruption was very shallow, originating beneaththe crater at depths of less than 2 km and possibly lessthan 1 km. Geochemical study of volcanic fluids andtephra implies that the erupted materials came from ashallow hydrothermal system below the summit. This
inference is consistent with analysis of the single VLP
event (suggesting opening at 0.3–1 km depth) and models for geodetic deformation. The earthquake hypocenter
locations (Kato et al. 2015) are 1–2 km deeper than thedepth of the source of the phreatic eruption estimatedfrom other methods. This inconsistency may result fromuncertainty regarding the shallow seismic-velocity structure. Improved seismic-velocity mapping of Mt. Ontake
would allow better hypocenter location beneath the summit area.

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before eruption. In the case of the 2014 eruption of Mt.
Ontake, rapid growth of tremor activity (Fig. 3 of Ogiso
et al. 2015) and tilting (Fig. 7 of Takagi and Onizawa
2016) were observed about 7 min before the eruption.
From the standpoint of public safety, it is therefore useful to consider options for communicating very shortterm warning of phreatic eruptions. Given the short timeframe, warnings may need to be generated automaticallyby means of an objective algorithm and communicateddirectly to the public by devices such as sirens or cellulardevices.
Longer-term “precursors” such as changes in groundlevel (Murase et al. 2016) and helium-isotope ratios (Sanoet al. 2015) have also been documented at Mt. Ontake.
However, given the long-term (annual to decadal) evolution of these phenomena, and the inherent intermittencyof the data, these observations are not useful for nearreal-time forecasting. They can be very useful for retrospective process understanding.
There were also midterm (days) changes in seismic activity prior to the 2014 eruption of Mt. Ontake.
The implications of the midterm seismic changes wereregarded as ambiguous in real time. No complementary
geodetic deformation was observed, but retrospective
analysis of geodetic data (Miyaoka and Takagi 2016) confirmed that minor near-summit deformation did occur.
An improved local geodetic network or analysis of InSAR
data might have facilitated near-real-time interpretationof midterm seismic and geodetic changes.

Since the 2014 eruption, the crater area has deflated ata nearly constant rate (Fig. 3). InSAR analysis (Narita andMurakami 2016) shows that ongoing deflation at a rateof 2–3 × 105 m3 per year is also shallow-rooted (0.5 km
depth) and localized, consistent with continued depressurization of the hydrothermal system. Post-eruption
heat-discharge rates and the corresponding water mass
fluxes are larger (order 104–105 tons per day, Terada2014) than the deflation volume estimated with geodetic
observations, suggesting continued contribution of additional hydrothermal fluids from larger, deeper sourceareas. A low-resistivity structure mapped by Sabry andMogi (2016) extends beneath both the crater and eastern flank and suggests a possible connection to a largerhydrothermal system. The leveling survey by Murase
et al. (2016) documents uplift and subsidence on the eastern flank between 2007 and 2014, possibly related to ahydrothermal system beneath the eastern flank.
Suggestions for future disaster mitigation

Most of the precursors that we rely on to forecast volcanic eruptions depend upon detection of fluid movement or its consequences. Because of the shallow source
region and the low viscosity of the moving fluids, phreatic eruptions such as the 2014 eruption of Mt. Ontake
can occur with little warning; fluid movement may begin
only minutes in advance of eruption. However, obvious and potentially useful seismic and geodetic precursors may be detected tens of seconds to tens of minutes

1500

GNSS-1

2000

1500

2500

Jigokudani
GNSS-2

Eruption

Baseline length between GNSS-1 and 2 (8622.20m)

Extensioncm

year
Fig. 3 Temporal change in baseline length between GNSS‑1 (Ochiai‑Karatani station) and GNSS‑2 (Tanohara station); GNSS plot courtesy of theJapan Meteorological Agency

6Earth, Planets and Space
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Page 7 of 8vent, the vent-sourced lahar traveled about 5 km, sufficient distance to threaten nearby communities and affectthe pH and turbidity of water supplies (Sasaki et al. 2016);
thus, there is also potential for more distal damage.

It is possible that magmatic intrusion prior to 1979 initiated the recent sequence of phreatic eruptions (1979,
1991, 2007, 2014) by adding heat and magmatic volatiles
to the hydrothermal system. This speculation is consistent with increasing 3He/4He ratios observed since 1981
in proximal samples (Sano et al. 2015). The nature ofgeodetic deformation in 2007 suggests additional intrusion of new magma at mid-crustal depths of approximately 5 km. Although none of the recent eruptions haveejected juvenile material, the possibility of magma eruption remains, and the officials concerned with forecastingand public safety must consider the possibility that purely
phreatic eruptions will evolve into phreatomagmatic
eruptions.
It also seems possible, perhaps likely, that the recent
cycle of phreatic eruptions at Mt. Ontake (1979, 1991,
2007, 2014) reflects repeated pressurization and breaching of the shallow hydrothermal system. In the absenceof seismicity and associated fracturing, there is a fairly
universal tendency for permeability to decrease due tomineral precipitation and other hydrothermal-alteration
processes (Gleeson and Ingebritsen 2016). Thus, diminished fumarolic activity may reflect a period of sealing
and pressurization of the shallow hydrothermal system;
fractures begin to open or shear when fluid pressure issufficiently elevated.
Monitoring and forecasting capability might be
improved by deploying additional geophysical- andgeochemical-monitoring capabilities in the near vicinityof the summit crater. Magnetometers will be deployed
to monitor the magnetic total field, which may help tomap ongoing hydrothermal alteration and temperaturechanges at relatively shallow depths. Gas monitoring will
also help to understand the ongoing hydrothermal processes. Generating continuous long-term time series will
require sustained effort in a remote near-summit environment that is characterized by extreme weather anddischarge of high-temperature, low-pH hydrothermal
fluids.
The 2014 eruption provides some lessons regarding
public safety in the context of phreatic eruption. Most
fatalities were related to impact ejecta. At Mt. Ontake,there was a window of several tens of seconds before
the first ballistic impacts (Kaneko et al. 2016), which
achieved energy on the order of 104 J (Tsunematsu et al.
2016). Thus, the public should be aware of nearby structures or natural features that can provide shelter fromejecta. The temperature of the PDC in this specific casewas 30–100 °C (Oikawa et al. 2016), but could potentiallyhave been higher because the temperature at the hydrothermal source is estimated to have been 270–280 °C(Ikehata and Maruoka 2016). Whereas the fatalities
caused by ballistic impact occurred within 1 km of the

Author details
1
Graduate School of Environmental Studies, Nagoya University, Nagoya,Japan. 2 Geological Survey of Japan, Advanced Industrial, Science and Technol‑ogy, Tsukuba, Japan. 3 Faculty of Science, Hokkaido University, Sapporo, Japan.
4
United States Geological Survey, Menlo Park, CA, USA.
Received: 12 October 2016 Accepted: 14 October 2016

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Tanaka R (2016) Volcanic plume measurements using a UAV for the2014 Mt. Ontake eruption. Earth Planets Space 68:49. doi:10.1186/
s40623‑016‑0418‑0
Murase M, Kimata F, Yamanaka Y, Horikawa S, Matsuhiro K, Matsushima T,
Mori H, Ohkura T, Yoshikawa S, Miyajima R, Inoue H, Mishima T, Sonoda T,
Uchida K, Yamamoto K, Nakamichi H (2016) Preparatory process preced‑
ing the 2014 eruption of Mount Ontake volcano, Japan: insights fromprecise leveling measurements. Earth Planets Space 68:9. doi:10.1186/
s40623‑016‑0386‑4
Nakamichi H, Kumagai H, Nakano M, Okubo M, Kimata F, Ito Y, Obara K (2009)
Source mechanism of a very‑long‑period event at Mt Ontake, central
Japan: response of a hydrothermal system to magma intrusion beneaththe summit. J Volcanol Geotherm Res 187:167–177
Narita S, Murakami M (2016) Deflation source after the September 2014
eruption of Ontake Volcano, Japan detected by ALOS2/PALSAR2 InSAR.
Abstract, Japan Geoscience Union Meeting 2016, SVC47‑26*Corresponding author: Jose van den IJssel, j.a.a.vandenijssel@tudelft.nl
7

The Phreatic Eruption of Mt. Ontake Volcano in 2014
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Ogiso M, Matsubyashi H, Yamamoto T (2015) Descent of tremor source loca‑
tions before the 2014 phreatic eruption of Ontake volcano, Japan. Earth
Planets Space 67:206. doi:10.1186/s40623‑015‑0376‑yOikawa T (2008) Reinvestigation of the historical eruption and fumarolicactivity records at Ontake Volcano, central Japan. Bull Geol Surv Japan59(5–6):203–210
Oikawa T, Suzuki Y, Chiba T, Kishimoto H, Okuno M, Ishizuka O (2015) Holocene
eruption hisotry of the Ontake Volcano, central Japan. Abstracts, Fall
Meeting of the Volcanological Society of Japan, 102 (in Japanese)
Oikawa T, Yoshimoto M, Nakada S, Maeno F, Komori J, Shimano T, Takeshita
Y, Ishizuka Y, Ishimine Y (2016) Reconstruction of the 2014 eruption
sequence of Ontake Volcano from recorded images and interviews. Earth
Planets Space 68:79. doi:10.1186/s40623‑016‑0458‑5
Sabry AA, Mogi T (2016) Three‑dimensional resistivity modeling of GREATEM
survey data from Ontake Volcano, northwest Japan. Earth Planets Space
68:76. doi:10.1186/s40623‑016‑0433‑z
Sano Y, Kagoshima T, Takahata N, Nishio Y, Roullearu E, Pinti DL, Fischer P (2015)
Ten‑year helium anomaly prior to the 2014 Mt Ontake eruption. Sci Rep
5:13069. doi:10.1038/srep13069
Sasaki H, Chiba T, Kishimoto H, Naruke S (2016) Characteristics of thesyneruptive‑spouted type lahar generated by the September 2014 erup‑tion of Mount Ontake, Japan. Earth Planets Space 68:141. doi:10.1186/
s40623‑016‑0516‑z

Takagi A, Onizawa S (2016) Shallow pressure sources associated with the 2007
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Takarada A, Oikawa T, Furukawa R, Hoshizumi H, Itoh J, Geshi N, Miyagi I (2016)
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meeting of Volcanological Society of Japan, UP‑24 (in Japanese)
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T (2016) Monitoring eruption activity using temporal stress changes
at Mount Ontake volcano. Nat Commun 7:10797. doi:10.1038/
ncomms10797
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Estimation of ballistic block landing energy during 2014 Mount Ontake
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Ontake volcano in 2007 and 2014. Geophys Res Lett 42:6982–6988. doi:1
0.1002/2015GL065165

8

Earth, Planets and SpaceFRONTIER LETTER Open AccessPreparatory and precursory processes leading up to the 2014 phreatic
eruption of Mount Ontake, Japan
Aitaro Kato*, Toshiko Terakawa, Yoshiko Yamanaka, Yuta Maeda, Shinichiro Horikawa,
Kenjiro Matsuhiro and Takashi Okuda
Earth, Planets and Space 2015, 67:111 DOI: 10.1186/s40623-015-0288-x
Received: 15 April 2015, Accepted: 2 July 2015, Published: 16 July 2015

Abstract
We analyzed seismicity linked to the 2014 phreatic eruption of Mount Ontake, Japan, on 27 September 2014. We first relocatedshallow volcano tectonic (VT) earthquakes and long-period (LP) events from August to September 2014. By applying a matched-filter
technique to continuous waveforms using these relocated earthquakes, we detected numerous additional micro-earthquakes
beneath the craters. The relocated VT earthquakes aligned on a near-vertical plane oriented NNW–SSE, suggesting they occurred
around a conduit related to the intrusion of magmatic–hydrothermal fluids into the craters. The frequency of VT earthquakes
gradually increased from 6 September 2014 and reached a peak on 11 September 2014. After the peak, seismicity levels remained
elevated until the eruption. b-values gradually increased from 1.2 to 1.7 from 11 to 16 September 2014 then declined gradually anddropped to 0.8 just before the eruption. During the 10-min
period immediately preceding the phreatic eruption, VT
earthquakes migrated in the up-dip direction as well aslaterally along the NNW–SSE feature. The migrating seismicity
coincided with an accelerated increase of pre-eruptive tremor
amplitude and with an anomalous tiltmeter signal thatindicated summit upheaval. Therefore, the migrating seismicity
suggests that the vertical conduit was filled with pressurized
fluids, which rapidly propagated to the surface during the final
10 min before the eruption.

Keywords: Mount Ontake; 2014 phreatic eruption; Precursor;

Earthquake; Relocation; Matched-filter technique;b-value

Figure 1*Corresponding author: Aitaro Kato, aitaro@seis.nagoya-u.ac.jp

FULL PAPER

Open Access

Descent of tremor source locations before the 2014
phreatic eruption of Ontake volcano, Japan
Masashi Ogiso*, Hirotoshi Matsubayashi and Tetsuya Yamamoto
Earth, Planets and Space 2015, 67:206 DOI: 10.1186/s40623-015-0376-y
Received: 12 August 2015, Accepted: 16 December 2015, Published: 29 December 2015

Abstract
On 27 September 2014, Ontake volcano, in central Japan, suddenly erupted without precursory activity. We estimated andtracked the source locations of volcanic tremor associated with the eruption at high temporal resolution, using a method
based on the spatial distribution of tremor amplitudes. Although the tremor source locations were not well constrained in
depth, their epicenters were well located beneath the erupted crater and the summit. Tremor sources were seen to descend
approximately 2 km over a period of several minutes prior to the beginning of the eruption. Detailed analysis of the timeseries of tremor amplitudes suggests that this descent is a robust feature. Our finding may be an important constraint formodeling the 2014 eruption of Ontake volcano as well as for monitoring activities on this and other volcanoes.
Eruption (Sep. 27)

Keywords: Tremor location; Migration; Mechanism of200

phreatic eruption; Volcano monitoring; Ontake
volcano483

Frequency

150
100
50
0

2014/08

2014/09

Figure 1*Corresponding author: Masashi Ogiso, mogiso@mri-jma.go.jp
92014/10

The Phreatic Eruption of Mt. Ontake Volcano in 2014

FULL PAPER

Open AccessPreparatory process preceding the 2014 eruption of Mount Ontake
volcano, Japan: insights from precise leveling measurements
Masayuki Murase*, Fumiaki Kimata, Yoshiko Yamanaka, Shinichiro Horikawa, Kenjiro Matsuhiro,
Takeshi Matsushima, Hitoshi Mori, Takahiro Ohkura, Shin Yoshikawa, Rikio Miyajima, Hiroyuki Inoue,
Taketoshi Mishima, Tadaomi Sonoda, Kazunari Uchida, Keigo Yamamoto and Harushisa Nakamichi
Earth, Planets and Space 2016, 68:9 DOI: 10.1186/s40623-016-0386-4
Received: 6 October 2015, Accepted: 13 January 2016, Published: 22 January 2016

Abstract

Keywords: Mount Ontake volcano; Preparatory process; Precise leveling survey; Vertical(a)

Deformations for each survey period (2006-2014)
Apr. 2009- Oct. 2014

(One division of vertical scale represents 5 mm)

Preparatory activity preceding the 2014 eruption of Mount Ontake volcano was estimatedfrom vertical deformation detected using a precise leveling survey. Notable uplift (2006–2009)and subsidence (2009–2014) were detected on the eastern flank of the volcano. We estimatedpressure source models based on the vertical deformation and used these to infer preparatory
process preceding the 2014 eruption. Our results suggest that the subsidence experienced
between 2009 and 2014 (including the period of the 2014 eruption) occurred as a result of asill-like tensile crack with a depth of 2.5 km. This tensile crack might inflate prior to theeruption and deflate during the 2014 activity. A two-tensile-crack model was used to explain
uplift from 2006 to 2009. The geometry of the shallow crack was assumed to be the same as
the sill-like tensile crack. The deep crack was estimated to be 2 km in length, 4.5 km in width,and 3 km in depth. Distinct uplifts began on the volcano flanks in 2006 and were followed byseismic activities and a small phreatic eruption in 2007. From the partially surveyed levelingdata in August 2013, uplift might continue until August 2013 without seismic activity in thesummit area. Based on the uplift from 2006 to 2013, magma ascended rapidly beneath thesummit area in December 2006, and deep and shallow tensile cracks were expanded between
2006 and 2013. The presence of expanded cracks between 2007 and 2013 has not beeninferred by previous studies. A phreatic eruption occurred on 27 September 2014, and,following this activity, the shallow crack may have deflated.

5mm
May 2008- Apr.2009

Apr. 2007- May 2008

Apr. 2006- Apr.2007

0

(b)

5

10

15

20

Relative elevation referred to BM34

(m)
300
200
100
0

0

5

deformation; Pressure source

10
Distance from BM34 (km)15

20

Figure 2*Corresponding author: Masayuki Murase, murase@chs.nihon-u.ac.jp

FULL PAPER

Open Access

Volcanic plume measurements using a UAV for the 2014 Mt. Ontake
eruption
Toshiya Mori*, Takeshi Hashimoto, Akihiko Terada, Mitsuhiro Yoshimoto, Ryunosuke Kazahaya,
Hiroshi Shinohara and Ryo Tanaka
Earth, Planets and Space 2016, 68:49 DOI: 10.1186/s40623-016-0418-0
Received: 30 November 2015, Accepted: 24 February 2016, Published: 22 March 2016

Abstract
A phreatic eruption of Mt. Ontake, Japan, started abruptly on September 27, 2014, and caused the worst volcanic calamity in recent
70 years in Japan. We conducted volcanic plume surveys using an electric multirotor unmanned aerial vehicle to elucidate theconditions of Mt. Ontake’s plume, which is flowing over 3000 m altitude. A plume gas composition, sulfur dioxide flux and thermal
image measurements and a particle sampling were carried out using the unmanned aerial vehicle for three field campaigns on
November 20 and 21, 2014, and June 2, 2015. Together with the results of manned helicopter and aircraft observations, we revealed
that the plume of Mt. Ontake was not directly emitted from the magma but was influenced by hydrothermal system, and observed
SO2/H2S molar ratios were decreasing after the eruption. High SO2 flux of >2000 t/d
observed at least until 20 h after the onset of the eruption implies significant input ofbmagmatic gas and the flux quickly decreased to about 130 t/d in 2 months. In contrast,
H2S fluxes retrieved using SO2/H2S ratio and SO2 flux showed significantly high level of
700–800 t/d, which continued at least between 2 weeks and 2 months after the eruption.
This is a peculiar feature of the 2014 Mt. Ontake eruption. Considering the trends of theflux changes of SO2 and H2S, we presume that majority of SO2 and H2S are supplied,
respectively, from high-temperature magmatic fluid of a deep origin and fromhydrothermal system. From the point of view of SO2/H2S ratios and fumarolictemperatures, the plume degassing trend after the 2014 eruption is following the similarcourse as that after the 1979 eruptions, and we speculate the 2014 eruptive activity willcease slowly similar to the 1979 eruption.

Keywords: Mt. Ontake; The 2014 eruption; Phreatic eruption; Volcanic plume; Sulfur
dioxide; Hydrogen sulfide; Volcanic gas flux; UAV; MultiGAS*Corresponding author: Toshiya Mori, mori@eqchem.s.u-tokyo.ac.jp
10

Figure 1

Earth, Planets and Space

FULL PAPER

Open Access2014 Mount Ontake eruption: characteristics of the phreatic eruption
as inferred from aerial observations
Takayuki Kaneko*, Fukashi Maeno and Setsuya Nakada
Earth, Planets and Space 2016, 68:72 DOI: 10.1186/s40623-016-0452-y
Received: 20 November 2015, Accepted: 20 April 2016, Published: 3 May 2016

Abstract
The sudden eruption of Mount Ontake on September 27, 2014, led to a tragedy that caused more than 60 fatalities including missing
persons. In order to mitigate the potential risks posed by similar volcano-related disasters, it is vital to have a clear understanding of
the activity status and progression of eruptions. Because the erupted material was largely disturbed while access was strictly
prohibited for a month, we analyzed the aerial photographs taken on September 28. The results showed that there were three large
vents in the bottom of the Jigokudani valley on September 28. The vent in the center was considered to have been the main ventinvolved in the eruption, and the vents on either side were considered to have been formed by non-explosive processes. The
pyroclastic flows extended approximately 2.5 km along the valley at an average speed of 32 km/h. The absence of burned or fallentrees in this area indicated that the temperatures and destructive forces associated withthe pyroclastic flow were both low. The distribution of ballistics was categorized into fourzones based on the number of impact craters per unit area, and the furthest impact crater
was located 950 m from the vents. Based on ballistic models, the maximum initial velocityof the ejecta was estimated to be 111 m/s. Just after the beginning of the eruption, very
few ballistic ejecta had arrived at the summit, even though the eruption plume had risen
above the summit, which suggested that a large amount of ballistic ejecta was expelledfrom the volcano several tens-of-seconds after the beginning of the eruption. This initialperiod was characterized by the escape of a vapor phase from the vents, which thencaused the explosive eruption phase that generated large amounts of ballistic ejecta via
sudden decompression of a hydrothermal reservoir.

Keywords: Volcano; Aerial observation; Ballistics; Impact crater; Ontake; Phreatic
eruption

Figure 2*Corresponding author: Takayuki Kaneko, kaneko@eri.u-tokyo.ac.jp

FULL PAPER

Open AccessThree-dimensional resistivity modeling of GREATEM survey
data from Ontake Volcano, northwest Japan
Sabry Abd Allah* and Toru Mogi
Earth, Planets and Space 2016, 68:76 DOI: 10.1186/s40623-016-0443-z
Received: 25 December 2015, Accepted: 13 April 2016, Published: 6 May 2016

Abstract
Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27, 2014. To study the
structure of Ontake Volcano and discuss the process of its phreatic eruption, which can help in future eruptions mitigation,
airborne electromagnetic (AEM) surveys using the grounded electrical-source airborne transient electromagnetic (GREATEM)
system were conducted over Ontake Volcano. Field measurements and data analysis were done by OYO Company under the
Sabo project managed by the Ministry of Land, Infrastructure, Transport and Tourism. Processed data and 1D resistivity models
were provided by this project. We performed numerical forward modeling to generate a three-dimensional (3D) resistivity
structure model that fits the GREATEM data where a composite of 1D resistivity models was used as the starting model. A 3D
electromagnetic forward-modeling scheme based on a staggered-gridfinite-difference method was modified and used to calculate theresponse of the 3D resistivity model along each survey line. We verified
the model by examining the fit of magnetic-transient responses
between the field data and 3D forward-model computed data. The
preferred 3D resistivity models show that a moderately resistive
structure (30–200 Ωm) is characteristic of most of the volcano, and wereable to delineate a hydrothermal zone within the volcanic edifice. Thishydrothermal zone may be caused by a previous large sector collapse.

Keywords: Airborne EM; 3D resistivity modeling; GREATEM survey;
Volcanic surveys

Figure 2*Corresponding author: Sabry Abd Allah, Sabry_eeaa@hotmail.com
11

The Phreatic Eruption of Mt. Ontake Volcano in 2014

FULL PAPER

Open Access

Reconstruction of the 2014 eruption sequence of Ontake Volcano
from recorded images and interviews
Teruki Oikawa*, Mitsuhiro Yoshimoto, Setsuya Nakada, Fukashi Maeno, Jiro Komori,Taketo Shimano, Yoshihiro Takeshita, Yoshihiro Ishizuka and Yasuhiro Ishimine
Earth, Planets and Space 2016, 68:79 DOI: 10.1186/s40623-016-0458-5
Received: 9 December 2015, Accepted: 29 April 2016, Published: 14 May 2016

Abstract
A phreatic eruption at Mount Ontake (3067 m) on September 27, 2014, led to 64 casualties, including missing people. In this paper,
we clarify the eruption sequence of the 2014 eruption from recorded images (photographs and videos obtained by climbers) andinterviews with mountain guides and workers in mountain huts. The onset of eruption was sudden, without any clear precursory
surface phenomena (such as ground rumbling or strong smell of sulfide). Our data indicate that the eruption sequence can bedivided into three phases. Phase 1: The eruption started with dry pyroclastic density currents (PDCs) caused by ash column collapse.
The PDCs flowed down 2.5 km SW and 2 km NW from the craters. In addition, PDCs moved horizontally by approximately 1.5 km
toward N and E beyond summit ridges. The temperature of PDCs at the summit area partially exceeded 100 °C, and an analysis ofinterview results suggested that the temperature of PDCs was mostly
in the range of 30–100 °C. At the summit area, there were violent
falling ballistic rocks. Phase 2: When the outflow of PDCs stopped, thealtitude of the eruption column increased; tephra with muddy rainstarted to fall; and ambient air temperature decreased. Fallingballistic rocks were almost absent during this phase. Phase 3: Finally,
muddy hot water flowed out from the craters. These models
reconstructed from observations are consistent with the phreatic
eruption models and typical eruption sequences recorded at similar
volcanoes.

Keywords: Ontake Volcano; Phreatic eruption; Eruption sequence;

Photo; Video; Interview; Pyroclastic density current; Ashcolumn collapse; Muddy rain; Lahar

Figure 2*Corresponding author: Teruki Oikawa, teruki-oikawa@aist.go.jp

FULL PAPER

Open Access

Reconstruction of a phreatic eruption on 27 September 2014 at
Ontake volcano, central Japan, based on proximal pyroclastic density
current and fallout deposits
Fukashi Maeno*, Setsuya Nakada, Teruki Oikawa, Mitsuhiro Yoshimoto, Jiro Komori,
Yoshihiro Ishizuka, Yoshihiro Takeshita, Taketo Shimano, Takayuki Kaneko and Masashi Nagai
Earth, Planets and Space 2016, 68:82 DOI: 10.1186/s40623-016-0449-6
Received: 1 December 2015, Accepted: 18 April 2016, Published: 17 May 2016

Abstract
The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan,
was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations werealso used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which arecharacterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30–50 wt% of fine ash (silt–clay component).
The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in therelatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C
was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As
observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect ofwater in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, andby applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of
the PDC at the summit area were estimated as 2 × 10−4–2 × 10−3 and 24–28
m/s, respectively. The tephra thinning trend in the proximal area shows asteeper slope than in similar-sized magmatic eruptions, indicating a large
tephra volume deposited over a short distance owing to the wet dispersal
conditions. The Ontake eruption provided an opportunity to examine thedeposits from a phreatic eruption with a complex eruption sequence thatreflects the effect of external water on the eruption dynamics.

Keywords: Phreatic eruption; Pyroclastic density current; Ontake volcano;Aggregate; Grain-size distribution; Particle concentration; Flow
speed

*Corresponding author: Fukashi Maeno, fmaeno@eri.u-tokyo.ac.jp
12

Figure 2

Earth, Planets and Space

FULL PAPER

Open Access

Estimation of ballistic block landing energy during 2014 Mount
Ontake eruption
Kae Tsunematsu*, Yasuhiro Ishimine, Takayuki Kaneko, Mitsuhiro Yoshimoto, Toshitsugu Fujii
and Koshun Yamaoka
Earth, Planets and Space 2016, 68:88 DOI: 10.1186/s40623-016-0463-8
Received: 30 November 2015, Accepted: 6 May 2016, Published: 25 May 2016

Abstract
The 2014 Mount Ontake eruption started just before noon on September 27, 2014. It killed 58 people, and five are still missing(as of January 1, 2016). The casualties were mainly caused by the impact of ballistic blocks around the summit area. It isnecessary to know the magnitude of the block velocity and energy to construct a hazard map of ballistic projectiles anddesign effective shelters and mountain huts. The ejection velocities of the ballistic
projectiles were estimated by comparing the observed distribution of the ballistic
impact craters on the ground with simulated distributions of landing positions underdvarious sets of conditions. A three-dimensional numerical multiparticle ballistic
model adapted to account for topographic effect was used to estimate the ejection
angles. From these simulations, we have obtained an ejection angle of γ = 20° fromvertical to horizontal and α = 20° from north to east. With these ejection angle
Gas flow effect range
conditions, the ejection speed was estimated to be between 145 and 185 m/s for a(Lf)previously obtained range of drag coefficients of 0.62–1.01. The order of magnitude
4of the mean landing energy obtained using our numerical simulation was 10 J.

Keywords: Ballistics; Mount Ontake; 3D multiparticle numerical model; Drag;
Topographic effectFigure 2*Corresponding author: Kae Tsunematsu, kae.tsunematsu@mfri.pref.yamanashi.jp

FULL PAPER

Open Access

Estimation of total discharged mass from the phreatic eruption of
Ontake Volcano, central Japan, on September 27, 2014
Shinji Takarada*, Teruki Oikawa, Ryuta Furukawa, Hideo Hoshizumi, Jun’ichi Itoh, Nobuo Geshi
and Isoji Miyagi
Earth, Planets and Space 2016, 68:138 DOI: 10.1186/s40623-016-0511-4
Received: 4 January 2016, Accepted: 11 July 2016, Published: 2 August 2016

Abstract
The total mass discharged by the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014, was estimatedusing several methods. The estimated discharged mass was 1.2 × 106 t (segment integration method), 8.9 × 105 t (Pyle’s
exponential method), and varied from 8.6 × 103 to 2.5 × 106 t (Hayakawa’s single isopach method). The segment integration
and Pyle’s exponential methods gave similar values. The single isopach method, however, gave a wide range of results
depending on which contour was used. Therefore, the total discharged mass of the 2014 eruption is estimated at between
8.9 × 105 and 1.2 × 106 t. More than 90 % of the total mass accumulated within the proximal area. This shows how important it
is to include a proximal area field survey for the total mass estimation of phreatic eruptions. A detailed isopleth mass
distribution map was prepared covering as far as 85 km from the source. The main ash-fall dispersal was ENE in the proximaland medial areas and E in the distal area. The secondary distribution lobes also extended to the S and NW proximally,reflecting the effects of elutriation ash and surge deposits from pyroclastic density currents during the phreatic eruption. The
total discharged mass of the 1979 phreatic eruption was also
calculated for comparison. The resulting volume of 1.9 × 106 t(using the segment integration method) indicates that it was
about 1.6–2.1 times larger than the 2014 eruption. The estimatedaverage discharged mass flux rate of the 2014 eruption was 1.7 ×
108 kg/h and for the 1979 eruption was 1.0 × 108 kg/h. One of thepossible reasons for the higher flux rate of the 2014 eruption is theoccurrence of pyroclastic density currents at the summit area.
Keywords: Ontake; 2014 Eruption; Phreatic; Discharged mass;
Ash fall; Distribution; 1979 Eruption; Flux rate
Figure 1*Corresponding author: Shinji Takarada, s-takarada@aist.go.jp
13

The Phreatic Eruption of Mt. Ontake Volcano in 2014

LETTER 

Open Access

Source mechanism of a VLP event immediately before the 2014
eruption of Mt. Ontake, Japan
Yuta Maeda*, Aitaro Kato, Toshiko Terakawa, Yoshiko Yamanaka, Shinichiro Horikawa,
Kenjiro Matsuhiro and Takashi Okuda
Earth, Planets and Space 2015, 67:187 DOI: 10.1186/s40623-015-0358-0
Received: 16 September 2015, Accepted: 12 November 2015, Published: 24 November 2015

Abstract
The phreatic eruption of Mt. Ontake in 2014 was preceded for 25 s by a very long period (VLP) seismic event recorded at one
broadband and three short-period seismic stations located within 5000 m of the summit. We performed waveform inversionof the event within a frequency band of 0.1–0.5 Hz that pointed to an ENE-WSW opening (NNW-SSE striking) subverticaltensile crack at a depth of 300–1000 m beneath the region of the eruptive vents. This crack orientation is consistent withalignments of volcano-tectonic (VT) earthquake hypocenters and eruptive vents as well as normal faulting (E-W tension) focal
mechanisms of the VT earthquakes. We interpreted these results as follows: the
VLP source crack was one of a group of preexisting faults that was opened
immediately before the eruption because of the passage of ascending gas from
0
V.ONTAdepth to the surface.
(2243 m)Keywords: Phreatic eruption; Mt. Ontake; VLP event; Waveform inversion

Velocity (10−6 m s−1)

−5
0
−5
0
−5
0
−5

ROPW
(2830 m)V.ONTN(3019 m)NU.NGR1(4344 m)
11:52:00
11:52:30
11:53:00
Time (JST) on 27 September 2014

Figure 2*Corresponding author: Yuta Maeda, maeda@seis.nagoya-u.ac.jp

LETTER 

Open Access

Groundwater pressure changes and crustal deformation
before and after the 2007 and 2014 eruptions of Mt. Ontake
Naoji Koizumi*, Tsutomu Sato, Yuichi Kitagawa and Tadafumi Ochi
Earth, Planets and Space 2016, 68:48 DOI: 10.1186/s40623-016-0420-6
Received: 27 November 2015, Accepted: 26 February 2016, Published: 17 March 2016

Abstract
Volcanic activity generally causes crustal deformation, which sometimes induces groundwater changes, and both of these
phenomena are sometimes detected before volcanic eruptions. Therefore, investigations of crustal deformation andgroundwater changes can be useful for predicting volcanic eruptions. The Geological Survey of Japan, National Institute ofAdvanced Industrial Science and Technology, has been observing groundwater pressure at Ohtaki observatory (GOT) since1998. GOT is about 10 km southeast of the summit of Mt. Ontake. During this observation period, Mt. Ontake has erupted
twice, in 2007 and in 2014. Before the 2007 eruption, the groundwater pressure at GOT clearly dropped, but it did not change
before or after the 2014 eruption. These observations are consistent with the crustal deformation observed by Global
Navigation Satellite System stations of the Geospatial Information Authority of Japan. The difference between the 2007 and2014 eruptions can be explained if a relatively large magma intrusion occurred before the 2007 eruption but no or a small
magma intrusion before the 2014 eruption.

Keywords: Mt. Ontake; Groundwater; Crustal deformation; Volcanic eruption;
Magma; Precursor

Figure 2*Corresponding author: Naoji Koizumi, koizumi.n@ses.usp.ac.jp
14Earth, Planets and SpaceLETTER 

Open Access

Detection of crustal deformation prior to the 2014 Mt. Ontake
eruption by the stacking method
Kazuki Miyaoka* and Akimichi Takagi
Earth, Planets and Space 2016, 68:60 DOI: 10.1186/s40623-016-0439-8
Received: 16 September 2015, Accepted: 6 April 2016, Published: 14 April 2016

Abstract
The phreatic eruption of Mt. Ontake in central Japan occurred in September 27, 2014. No obvious crustal deformation wasobserved prior to the eruption, and the magnitudes of other precursor phenomena were very small. In this study, we used thestacking method to detect crustal deformation prior to theeruption. The stacking method is a technique to improvebathe signal-to-noise ratio by stacking multiple records ofcrustal deformation. We succeeded in detecting a slightBB
spherical
crustal deformation caused by a volume change in thetensileA
A
pressure
crack
shallow region beneath the volcano’s summit from 1 monthsourcesourceprior to the eruption. We also detected a slight crustal
deformation that may have been caused by a volume
increase in the deep region from one and a half months
before the eruption. The magnitude of the volume changeAA
in the shallow region did not differ significantly in the 2014
A
A+B
B
eruption compared to the volume change during the small
B
Inverting -B
Stacking
Mt. Ontake eruption in 2007, and the volume change in the
Stacking
deep region was rather smaller in 2014 than in 2007.
A + ( -B)

Keywords: Mt. Ontake; Precursor phenomena; Crustal
deformation; Stacking method; GNSS

Figure 2*Corresponding author: Kazuki Miyaoka, miyaoka@mri-jma.go.jp

LETTER 

Open Access

Mineralogical study on volcanic ash of the eruption on September 27,
2014 at Ontake volcano, central Japan: correlation withporphyry copper systems
Yusuke Minami*, Takumi Imura, Shintaro Hayashi and Tsukasa Ohba
Earth, Planets and Space 2016, 68:67 DOI: 10.1186/s40623-016-0440-2
Received: 3 December 2015, Accepted: 8 April 2016, Published: 26 April 2016

Abstract
The volcanic ash of the eruption on September 27, 2014 at Ontake volcano consists mostly of altered rock fragments. The ash
contains partly altered volcanic rock fragments consisting of primary igneous minerals (plagioclase, orthopyroxene,
titanomagnetite, and feldspars) and volcanic glass accompanied by alteration minerals to some extents, and contains nojuvenile fragments. These features indicate that the eruption was a non-juvenile hydrothermal eruption that was derived fromthe hydrothermal system developed under the crater. The major minerals derived from hydrothermal alteration zones are
silica mineral, kaolin-group mineral, smectite, pyrophyllite, muscovite, alunite, anhydrite, gypsum, pyrite, K-feldspar, albite,and rutile. Minor chlorite, biotite, and garnet are accompanied. Five types of alteration mineral associations are identified fromobservations on individual ash particles: silica–pyrite, silica–pyrite ± alunite ± kaolin, silica–pyrophyllite–pyrite, silica–
muscovite ± chlorite, and silica–K-feldspar ± albite ± garnet ± biotite. The associations indicate development of advanced
argillic, sericite, and potassic alteration zones under the crater. Occurrence ofanhydrite veinlet and the set of alteration zones indicate hydrothermal alteration
zones similar to late-stage porphyry copper systems. Comparing the mineral
associations with the geologic model of the late-stage porphyry copper systems, thesource depths of mineral associations are estimated to range from near surface to>2 km. The depths of advanced argillic alteration, sericite, and potassic zones are 0
to ~2, ~1.5 to ~2, and >2 km, respectively.

Keywords: Hydrothermal eruption; Volcano-hydrothermal system; Acid-sulfatehydrothermal system; Ontake volcano

Figure 1*Corresponding author: Yusuke Minami, d9514007@wm.akita-u.ac.jp
15

The Phreatic Eruption of Mt. Ontake Volcano in 2014

LETTER 

Open Access

Sulfur isotopic characteristics of volcanic products from theSeptember 2014 Mount Ontake eruption, JapanKei Ikehata* and Teruyuki Maruoka
Earth, Planets and Space 2016, 68:116 DOI: 10.1186/s40623-016-0496-z
Received: 29 November 2015, Accepted: 22 June 2016, Published: 13 July 2016

Abstract
Components and sulfur isotopic compositions of pyroclastic materials from the 2014 Mt. Ontake eruption were investigated.
The volcanic ash samples were found to be composed of altered volcanic fragments, alunite, anhydrite, biotite, cristobalite,
gypsum, ilmenite, kaolin minerals, native sulfur, orthopyroxene, plagioclase, potassium feldspar, pyrite, pyrophyllite, quartz,
rutile, and smectite, and most of these minerals were likely derived from the acidic alteration zones of Mt. Ontake. The
absence of juvenile material in the eruptive products indicates that the eruption was phreatic. The sulfur isotopic
compositions of the water-leached sulfate, hydrochloric acid-leached sulfate, acetone-leached native sulfur, and pyrite of thesamples indicate that these sulfur species were produced by disproportionation of magmatic SO2 in the hydrothermal systemat temperatures of 270–281 °C. This temperature range is consistent with thatinferred from the hydrothermal mineral assemblage (e.g., pyrophyllite and rutile) in
the 2014 pyroclastic materials (200–300 °C). Except for the sulfur isotopic
compositions of anhydrite, which may have been altered by incorporation of sulfate
minerals in a fumarolic area with lower sulfur isotopic values into the undergroundmaterials during the 1979 eruption, no significant differences in the mineral
assemblages and sulfur isotopic compositions of the pyroclastic materials wereidentified between the products of the 2014 and 1979 Ontake phreatic eruptions,which suggests geochemical similarities in the underlying hydrothermal systems
before the 2014 and 1979 eruptions.

Keywords: Mount Ontake; Phreatic eruption; Sulfur isotopic composition;
Hydrothermal minerals

Figure 2*Corresponding author: Kei Ikehata, ikkei@geol.tsukuba.ac.jp

LETTER 

Open Access

Shallow pressure sources associated with the 2007 and2014 phreatic eruptions of Mt. Ontake, Japan
Akimichi Takagi* and Shin’ya Onizawa
Earth, Planets and Space 2016, 68:135 DOI: 10.1186/s40623-016-0515-0
Received: 9 March 2016, Accepted: 20 July 2016, Published: 29 July 2016

Abstract
We modeled pressure sources under Mount Ontake volcano, Japan, on the basis of global navigation satellite system (GNSS)
observations of ground deformation during the time period including the 2007 and 2014 phreatic eruptions. The total change
in volume in two sources below sea level in the period including the 2007 eruption was estimated from GNSS network
observations to be 6 × 106 m3. Additionally, data from a GNSS campaign survey yielded an estimated volume change of 0.28 ×
106 m3 in a shallower source just beneath the volcanic vents. The 2007 eruption may have been activated by magmatic activity
at depth. During the 2014 eruption, the volume change at depth was very small. However, tiltmeter data indicated inflationfrom a shallow source that began 7 min before the eruption, representing a volume change estimated to be 0.38 × 106 m3. We
infer that the potential for subsurface hydrothermal activity may have remained high after the 2007 eruption.

Keywords: Mt. Ontake; Phreatic eruption; Pressure

GSI0281 − GSI0988 the 2007 eruption

source; GNSS; Tiltmeterthe 2011 off the Pacificcoast of Tohoku EQ.
the 2014 eruptionm19096.48
19096.46
19096.44
19096.42

2001

200

2002

2003

2004

2005

2006

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2007

2008

2009

2010

2011

2012

2013

2014

2015

Daily number of volcanic earthquakes

N

150
100
50
0

2001

Figure 2*Corresponding author: Akimichi Takagi, atakagi@mri-jma.go.jp
16Earth, Planets and SpaceLETTER 

Open Access

Characteristics of the syneruptive-spouted type lahar generated by
the September 2014 eruption of Mount Ontake, Japan
Hisashi Sasaki*, Tatsuro Chiba, Hiroshi Kishimoto and Shino Naruke
Earth, Planets and Space 2016, 68:141 DOI: 10.1186/s40623-016-0516-z
Received: 17 December 2015, Accepted: 28 July 2016, Published: 9 August 2016

Abstract
Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash
falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated
by water released from within these craters, we refer to this lahar as a “syneruptive-spouted type lahar” in this study. The lahar of the2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake
breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream
from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 105 m3.
We have reviewed other representative syneruptive-spouted type lahars that have been reported inJapan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had thelongest runout distance and largest volume of all cases studied. The mineral assemblage identified
from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption.
Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of thesurface). The syneruptive-spouted type lahar deposits are therefore also considered to haveoriginated from shallow depths. A syneruptive-spouted type lahar is a small-scale phenomenon thatcauses little direct damage to infrastructure, but has long-term influence on water quality. Increases
in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of
Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term)
damage of syneruptive-spouted type lahars should be considered for hazard mapping and planningvolcanic disaster prevention measures.

Keywords: Phreatic eruption; Syneruptive-spouted type lahar; Muddy water; Water quality;
Ontake volcano

Figure 2*Corresponding author: Hisashi Sasaki, hss.sasaki@ajiko.co.jp

17

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Contents
Special issue “The phreatic eruption of Mt. Ontake volcano in 2014”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Koshun Yamaoka, Nobuo Geshi, Takeshi Hashimoto, S. E. Ingebritsen and Teruki Oikawa
Preparatory and precursory processes leading up to the 2014 phreatic eruption of Mount Ontake, Japan. . . . . . . . . Aitaro Kato, Toshiko Terakawa, Yoshiko Yamanaka, Yuta Maeda, Shinichiro Horikawa, Kenjiro Matsuhiro and Takashi Okuda
Descent of tremor source locations before the 2014 phreatic eruption of Ontake volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masashi Ogiso, Hirotoshi Matsubayashi and Tetsuya Yamamoto
Preparatory process preceding the 2014 eruption of Mount Ontake volcano, Japan: insights from precise levelingmeasurements. . . . . . . . . . . . . . . . . . . Masayuki Murase, Fumiaki Kimata, Yoshiko Yamanaka, Shinichiro Horikawa, Kenjiro Matsuhiro, Takeshi Matsushima,

Hitoshi Mori, Takahiro Ohkura, Shin Yoshikawa, Rikio Miyajima, Hiroyuki Inoue, Taketoshi Mishima, Tadaomi Sonoda,
Kazunari Uchida, Keigo Yamamoto and Harushisa Nakamichi

Volcanic plume measurements using a UAV for the 2014 Mt. Ontake eruption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toshiya Mori, Takeshi Hashimoto, Akihiko Terada, Mitsuhiro Yoshimoto, Ryunosuke Kazahaya,
Hiroshi Shinohara and Ryo Tanaka

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2014 Mount Ontake eruption: characteristics of the phreatic eruption as inferred from aerial observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takayuki Kaneko, Fukashi Maeno and Setsuya Nakada 11
Three-dimensional resistivity modeling of GREATEM survey data from Ontake Volcano, northwest Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sabry Abd Allah and Toru Mogi 11
Reconstruction of the 2014 eruption sequence of Ontake Volcano from recorded images and interviews. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Teruki Oikawa, Mitsuhiro Yoshimoto, Setsuya Nakada, Fukashi Maeno, Jiro Komori,Taketo Shimano, Yoshihiro Takeshita, Yoshihiro Ishizuka and Yasuhiro Ishimine

Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal
pyroclastic density current and fallout deposits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fukashi Maeno, Setsuya Nakada, Teruki Oikawa, Mitsuhiro Yoshimoto, Jiro Komori, Yoshihiro Ishizuka,
Yoshihiro Takeshita, Taketo Shimano, Takayuki Kaneko and Masashi Nagai

Estimation of ballistic block landing energy during 2014 Mount Ontake eruption. . . . . . . . . . . . . . . . . . Kae Tsunematsu, Yasuhiro Ishimine, Takayuki Kaneko, Mitsuhiro Yoshimoto, Toshitsugu Fujii and Koshun Yamaoka
Estimation of total discharged mass from the phreatic eruption of Ontake Volcano, central Japan, on September 27, 2014
. . . . . . . . . . . . . . . . . . . . . . Shinji Takarada, Teruki Oikawa, Ryuta Furukawa, Hideo Hoshizumi, Jun’ichi Itoh, Nobuo Geshi and Isoji Miyagi
Source mechanism of a VLP event immediately before the 2014 eruption of Mt. Ontake, Japan. . . . . . . . Yuta Maeda, Aitaro Kato, Toshiko Terakawa, Yoshiko Yamanaka, Shinichiro Horikawa, Kenjiro Matsuhiro and Takashi Okuda
Groundwater pressure changes and crustal deformation before and after the 2007 and 2014 eruptions of Mt. Ontake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Naoji Koizumi, Tsutomu Sato, Yuichi Kitagawa and Tadafumi Ochi
Detection of crustal deformation prior to the 2014 Mt. Ontake eruption by the stacking method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kazuki Miyaoka and Akimichi Takagi
Mineralogical study on volcanic ash of the eruption on September 27, 2014 at Ontake volcano, central Japan:
correlation with porphyry copper systems . . . . . . . . . . . . . . . . . . Yusuke Minami, Takumi Imura, Shintaro Hayashi and Tsukasa Ohba
Sulfur isotopic characteristics of volcanic products from the September 2014 Mount Ontake eruption, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kei Ikehata and Teruyuki Maruoka
Shallow pressure sources associated with the 2007 and 2014 phreatic eruptions of Mt. Ontake, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Akimichi Takagi and Shin’ya Onizawa
Characteristics of the syneruptive-spouted type lahar generated by the September 2014 eruption of Mount Ontake,Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hisashi Sasaki, Tatsuro Chiba, Hiroshi Kishimoto and Shino Naruke

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Cover image: Kato, A. et al., Earth, Planets and Space 2015, 67:111 DOI: 10.1186/s40623-015-0288-x