scholarly journals Deep Low‐Frequency Earthquakes Beneath the Hakone Volcano, Central Japan, and their Relation to Volcanic Activity

2019 ◽  
Vol 46 (20) ◽  
pp. 11035-11043 ◽  
Author(s):  
Y. Yukutake ◽  
Y. Abe ◽  
R. Doke
Keyword(s):  
Volcanic Activity ◽  
Low Frequency ◽  
Central Japan ◽  
Hakone Volcano

Five-Year Achievements of Volcano Program Promotion Panel

2020 ◽  
Vol 15 (2) ◽  
pp. 106-111
Author(s):  
Takahiro Ohkura ◽  
Kenji Nogami ◽  
◽  
Keyword(s):  
Volcanic Activity ◽  
Volcanic Eruption ◽  
Large Scale ◽  
Method Development ◽  
Low Frequency ◽  
Observation System ◽  
Event Tree ◽  
Volcanic Event ◽  
System Improvement ◽  
Observation Method

To mitigate a volcanic eruption disaster, it is important to forecast the transition of the disaster, which depends on the stage of the volcanic phenomena, in addition to forecasting the site, scale, and time of the volcanic activities. To make such forecasts, it is critical to elucidate the evolution of volcanic activity. Accordingly, the Volcano Program Promotion Panel has set the prioritized target as “to forecast volcanic eruption as a cause of disaster by clarifying the branching conditions and theories of volcanic activity and improving volcanic event tree.” The panel promoted a five-year study on the elucidation of volcanic phenomena, including low-frequency and large-scale ones, status of volcanic eruption fields, volcanic eruption modeling, observation method development, and observation system improvement. In this paper, an outline of the main results of this five-year study is presented.

scholarly journals Volcanic Unrest at Hakone Volcano after the 2015 phreatic eruption — Reactivation of a Ruptured Hydrothermal System?

2020 ◽  
Author(s):  
Kazutaka Mannen ◽  
Yuki Abe ◽  
Yasushi Daita ◽  
Ryosuke Doke ◽  
Masatake Harada ◽  
...  
Keyword(s):  
Hydrothermal System ◽  
Seismic Activity ◽  
Low Frequency ◽  
Magmatic Fluid ◽  
Volcanic Unrest ◽  
Hakone Volcano ◽  
Seismic Swarms ◽  
Cap Rock ◽  
Sealing Zone ◽  
Eruptive Period

Abstract Since the beginning of the 21st century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. In this paper, characteristics of the post-eruptive volcanic unrest that occurred in 2017 and 2019 are described, and changes in the hydrothermal system of the volcano caused by the eruption are discussed. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~3 km away from the center. The 2015 eruption established routes for steam from the hydrothermal system (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC), which was absent before the eruption. This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest, which may be interpreted as magmatic intrusion at shallow depth; however, no indicative seismic and geodetic signals were observed. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization of the hydrothermal system beneath the volcano center and suppressed seismic activity significantly. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–plastic transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

scholarly journals Volcanic unrest at Hakone volcano after the 2015 phreatic eruption: reactivation of a ruptured hydrothermal system?

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Kazutaka Mannen ◽  
Yuki Abe ◽  
Yasushi Daita ◽  
Ryosuke Doke ◽  
Masatake Harada ◽  
...  
Keyword(s):  
Hydrothermal System ◽  
Seismic Activity ◽  
Low Frequency ◽  
Magmatic Fluid ◽  
Volcanic Unrest ◽  
Hakone Volcano ◽  
Eruption Center ◽  
Brittle Ductile Transition ◽  
Seismic Swarms ◽  
Cap Rock

AbstractSince the beginning of the twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~ 2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~ 3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal system of the volcano could be composed of multiple sub-systems, each of which can host earthquake swarms and show independent volume changes. The 2015 eruption established routes for steam from the hydrothermal sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest episode; however, no magma supply was indicated by seismic and geodetic observations. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–ductile transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

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