2:30 PM - 2:45 PM
[SVC29-04] Volcanic unrests after the 2015 phreatic eruption of Hakone volcano and its hydrothermal system
Keywords:Hakone volcano, hydrothermal system, volcanic unrest, phreatic eruption, pore pressure, caprock
Since the beginning of the 21st century, volcanic unrests detected as increase of seismic activity, increase of baseline length across the edifice, and frequent deep low-frequency earthquakes have been observed at Hakone volcano every few years. The most intensive volcanic unrest culminated in a small phreatic eruption in 2015 as the first eruption in the history of observation. Even after the eruption, volcanic unrests have been observed in 2017 and 2019. In this presentation, we compare the pre- and post- eruptive unrests, and discuss the status of the hydrothermal system just below the eruption center.
Volcanic unrests of Hakone volcano start with a deep inflation (6-10 km) and deep low-frequency events around 20 km below the surface, followed by volcano tectonic earthquakes (< 6 km below the sea level). At the same time, the component ratio of magmatic to hydrothermal in the fumarole such as CO2/H2S and He/ H2S increases. In addition, abnormal steam emission from steam production wells in Owakudani can be observed. In fact, Volcano Alert Level 2 for Hakone Volcano is defined are crustal movement, seismic activity, and fumarolic anomalies that exceed the specified level. This series of changes can be interpreted as the supply of magma or magmatic fluids to the deep inflation source, which increase pore pressure of the hydrothermal system.
The post eruptive volcanic unrests occurred in 2017 and 2019 also accompanied deep inflation, volcanic tectonic earthquakes, changes in volcanic gas ratios, and deep low-frequency earthquakes. However, the frequency of volcano tectonic earthquake during the post-eruptive unrests was significantly lower especially in the shallow immediately beneath Owakudani. Also rise of magmatic/hydrothermal gas ratio was less significant during the post-eruptive unrest. In this presentation, we attribute these phenomena to the 2015 eruption, which ruptured the hydrothermal system near the surface (cap-rock) and at the bottom (sealing zone near the brittle-ductile boundary).
Recent magnetotelluric surveys showed that volcano tectonic earthquakes at Hakone Volcano occur within the hydrothermal system, and seismic activity during the unrests was interpreted as intrusion of fluid and the pore pressure rise within the hydrothermal system. The decrease in seismic activity observed in the post-eruptive unrest beneath Owakudani may be due to the destruction of the caprock by the 2015 eruption which allows no significant pressure rise within the hydrothermal system. If so, the future volcanic unrests may not be accompanied by the significant increase in seismic activity and local uplift seen in 2015 near Owakudani, until the healing of the caprock.
Temperature profile beneath Owakudani can be constrained from the mineral assemblage of bore hole samples and the bottoming of seismicity. Assuming the temperature profile and isoenthalpy of pore water through the hydrothermal system, vapor static pore pressure beneath the bottom of the caprock can be inferred. In a liquid-dominated system, unload of surface deposit can cause explosion of hydrothermal water near the boiling temperature just beneath the surface and eject surrounding material. This ejection causes further unload and explosion progresses downward (8). The fact that the 2015 eruption resulted in a very small magnitude with the explosion source only around 100 m deep may be attributed to the pressure profile described above. In addition, paucity of volcanic tremors and low-frequency earthquakes at Hakone volcano can be attributed to the vapor dominated hydrothermal system.
Assuming healing of caprock in progress and the steam-dominated hydrothermal system, risk of phreatic eruption near Owakudani may be low at present, and phreatic eruption larger than that of 2015 can be highly unlikely. On the other hand, since the deep inflation has continued even after the eruption, magmatic fluid can accumulate in deep and its release may cause major phreatic eruption in future as observed at Ontake volcano.
1. Mannen et al. Earth, Planets Sp. 70, 68 (2018).
2. Abe et al. Bull. Hot Springs. Res. Inst. 50, 1–18 (2018). (in Japanese)
3. Mannen et al. Earth, Planets Sp. in review
4. Yoshimura et al. Earth, Planets Sp. 70, 66 (2018).
5. Yukutake, Y. et al. J. Geophys. Res. Solid Earth 116, 1–13 (2011).
6. Doke et al. Earth, Planets Sp. 70, (2018).
7. Kobayashiet al. Earth Planet. Sci. Lett. 491, 244–254 (2018).
8. Browne and Lowless Earth-Sci. Rev. 52, 229-331 (2001)
Volcanic unrests of Hakone volcano start with a deep inflation (6-10 km) and deep low-frequency events around 20 km below the surface, followed by volcano tectonic earthquakes (< 6 km below the sea level). At the same time, the component ratio of magmatic to hydrothermal in the fumarole such as CO2/H2S and He/ H2S increases. In addition, abnormal steam emission from steam production wells in Owakudani can be observed. In fact, Volcano Alert Level 2 for Hakone Volcano is defined are crustal movement, seismic activity, and fumarolic anomalies that exceed the specified level. This series of changes can be interpreted as the supply of magma or magmatic fluids to the deep inflation source, which increase pore pressure of the hydrothermal system.
The post eruptive volcanic unrests occurred in 2017 and 2019 also accompanied deep inflation, volcanic tectonic earthquakes, changes in volcanic gas ratios, and deep low-frequency earthquakes. However, the frequency of volcano tectonic earthquake during the post-eruptive unrests was significantly lower especially in the shallow immediately beneath Owakudani. Also rise of magmatic/hydrothermal gas ratio was less significant during the post-eruptive unrest. In this presentation, we attribute these phenomena to the 2015 eruption, which ruptured the hydrothermal system near the surface (cap-rock) and at the bottom (sealing zone near the brittle-ductile boundary).
Recent magnetotelluric surveys showed that volcano tectonic earthquakes at Hakone Volcano occur within the hydrothermal system, and seismic activity during the unrests was interpreted as intrusion of fluid and the pore pressure rise within the hydrothermal system. The decrease in seismic activity observed in the post-eruptive unrest beneath Owakudani may be due to the destruction of the caprock by the 2015 eruption which allows no significant pressure rise within the hydrothermal system. If so, the future volcanic unrests may not be accompanied by the significant increase in seismic activity and local uplift seen in 2015 near Owakudani, until the healing of the caprock.
Temperature profile beneath Owakudani can be constrained from the mineral assemblage of bore hole samples and the bottoming of seismicity. Assuming the temperature profile and isoenthalpy of pore water through the hydrothermal system, vapor static pore pressure beneath the bottom of the caprock can be inferred. In a liquid-dominated system, unload of surface deposit can cause explosion of hydrothermal water near the boiling temperature just beneath the surface and eject surrounding material. This ejection causes further unload and explosion progresses downward (8). The fact that the 2015 eruption resulted in a very small magnitude with the explosion source only around 100 m deep may be attributed to the pressure profile described above. In addition, paucity of volcanic tremors and low-frequency earthquakes at Hakone volcano can be attributed to the vapor dominated hydrothermal system.
Assuming healing of caprock in progress and the steam-dominated hydrothermal system, risk of phreatic eruption near Owakudani may be low at present, and phreatic eruption larger than that of 2015 can be highly unlikely. On the other hand, since the deep inflation has continued even after the eruption, magmatic fluid can accumulate in deep and its release may cause major phreatic eruption in future as observed at Ontake volcano.
1. Mannen et al. Earth, Planets Sp. 70, 68 (2018).
2. Abe et al. Bull. Hot Springs. Res. Inst. 50, 1–18 (2018). (in Japanese)
3. Mannen et al. Earth, Planets Sp. in review
4. Yoshimura et al. Earth, Planets Sp. 70, 66 (2018).
5. Yukutake, Y. et al. J. Geophys. Res. Solid Earth 116, 1–13 (2011).
6. Doke et al. Earth, Planets Sp. 70, (2018).
7. Kobayashiet al. Earth Planet. Sci. Lett. 491, 244–254 (2018).
8. Browne and Lowless Earth-Sci. Rev. 52, 229-331 (2001)