International Workshop on the mechanism of Phreatic Eruption

The 2015 Hakone eruption: rupture of shallow vapor pocket triggered bymagma replenishment?
Kazutaka Mannen1
1 Hot Springs Research Institute of Kanagawa Prefecture
E-mail: mannen@onken.odawara.kanagawa.jp

Previous studies have indicated that the volcanic unrests of Hakone volcano were precededby inflation of deep source (~10 km deep) accompanying deep low frequency earthquakes(DLFE) (~20 km deep)1,2. The volcanic unrests, which include various phenomena such asearthquake swarms, inflation in shallow and blow out of steam wells, seem to be induced byincrease of pore fluid pressure within the hydrothermal system of the volcano (~5 km)2.
Approximately two months before the 2015 eruption of the volcano, when earthquake swarm
and blow out of a steam well started, very localized uplift (~ 200 m of diameter) in the future
eruption center was observed by satellite InSAR and its inflation sources was estimated tobe at around 150 m deep3,4. At this location and depth, we found resistive body by our CSAMT
survey5. The resistive body locatedwithin a smectite-rich conductive
layer, which is interpreted as thecap rock structure of the volcano6.
Since the location of resistive bodycoincident with the inflation sourceof before the eruption, it wasinterpreted as the vapor pocket. In
the vapor pocket, pores and cracks
within the cap rock structure wasfilled with vapor and inflated due toincrease of vapor flux during thevolcanic unrest. The 2015 eruption
was apparently triggered by ahydrothermal crack intrusion on
the morning of June 293,7. The
uppermost tip of the hydrothermal
crack may have been reached to
the vapor pocket; however, theFig. 1 Schematic model of the 2015 eruption centerand adjacent area. Beneath the eruption center
area, a vapor pocket is formed and the steam
migrates from the pocket to the surface through thecap rock. The open crack formed at 7:32 of 29 June2015 triggered the eruption, but the uppermost tip
seems not to have reached the surface.

International Workshop on the mechanism of Phreatic Eruption

water and solid particles originated from deeper than the vapor pocket were not recognized
during the eruption and the eruption seems to have been caused by the rupture of the vapor
pocket. Because the volume of the hydrothermal crack of the 2015 eruption (0.16 × 106 m3)
3,7 was comparable to that of the 2014 Ontake eruption (0.38 × 106 m3)8, the crack intrusion

had a potential to cause much larger and lethal eruption. The factor of controlling eruption
volume will be the next challenge to decipher the process of phreatic eruption.
After the 2015 eruption, a few volcanic unrests have been observed in Hakone volcano. Suchvolcanic unrests accompanied change in ratios of volcanic gases, inflation of deep source,and DLFE; however, very local uplift has never been observed and number of shallow
earthquakes beneath the 2015 eruption center is limited. These observations imply that the2015 eruption breached cap rock of the volcano and its hydrothermal system is no longer
able to be pressurized by the magma replenishment.
Main References
1.

Yukutake, Y., Abe, Y. & Doke, R. Deep Low-Frequency Earthquakes Beneath the Hakone
Volcano, Central Japan, and their Relation to Volcanic Activity. Geophys. Res. Lett. 1–9 (2019)
doi:10.1029/2019GL084357.

2.

Mannen, K. et al. Chronology of the 2015 eruption of Hakone volcano, Japan – geologicalbackground, mechanism of volcanic unrest and disaster mitigation measures during the crisis.
Earth, Planets Sp. 70, 68 (2018).

3.

Doke, R., Harada, M., Mannen, K., Itadera, K. & Takenaka, J. InSAR analysis for detecting theroute of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone Volcano,Japan. Earth, Planets Sp. 70, (2018).

4.

Kobayashi, T., Morishita, Y. & Munekane, H. First detection of precursory ground inflation of asmall phreatic eruption by InSAR. Earth Planet. Sci. Lett. 491, 244–254 (2018).

5.

Mannen, K. et al. Source constraints for the 2015 phreatic eruption of Hakone Volcano, Japan,
based on geological analysis and resistivity structure. Earth, Planets Sp. (2019)
doi:DOI :10.1186/s40623-019-1116-5.

6.

Yoshimura, R. et al. Resistivity characterisation of Hakone volcano, Central Japan, by threedimensional magnetotelluric inversion. Earth, Planets Sp. 70, 66 (2018).

7.

Honda, R. et al. Precursory tilt changes associated with a phreatic eruption of the Hakone volcano
and the corresponding source model. Earth, Planets Sp. 70, (2018).

8.

Takagi, A. & Onizawa, S. Shallow pressure sources associated with the 2007 and 2014 phreaticeruptions of Mt. Ontake, Japan. Earth, Planets Sp. 68, 135 (2016).