Quantitative relationship between plume emission and multiple deflations after the 2014 phreatic eruption at Ontake volcano, Japan

Abstract

The phreatic eruption of Mount Ontake in 2014 caused local-scale subsidence and a mass discharge of water-vapor plumes from vents. A previous study of InSAR data analysis modeled the local subsidence as a deflation of a shallow hydrothermal reservoir (~ 500 m beneath the vents), and speculated that it was associated with plume emission continuing just after the eruption. In addition, combination of the InSAR and GNSS data implies that another, deeper deflation source (~ 3-6 km beneath the vents) contributes to the baseline contraction of the GNSS data. In this study, we estimated daily mass flux of the emitting plumes using photographed images, and compared the temporal behavior of the discharged mass with that of deflation of the two sources in order to clarify their association. The temporal profiles of the shallow deflation volume and the discharge mass both show evidence of decay, but with different characteristics; the deflation volume progress was approximated by a single exponential decay with a long relaxation time (379-641 days), whereas the discharge mass displayed a sum of a linear trend and an exponential decay with shorter relaxation time (47 days). This discrepancy, along with GNSS data, suggests the contribution of a deep deflation source with a short relaxation time (20-40 days). Estimation of mass balance between the emitting plume and fluids discharged from both shallow and deep sources revealed that more than 70% of the discharged mass came from the deep source. Based on the estimated mass balance, phase state of the shallow reservoir was estimated as a single-phase, liquid-rich reservoir. The fast decay of the deep deflation may reflect rapid depressurization due to violent fluid discharge at the onset of the eruption. In contrast, the slow decay of the shallow deflation suggests that it had a minor role in the eruption. However, such a wet reservoir has the potential to induce volcanic hazard such as snow-melting lahar for future eruptions, requiring monitoring the volcano, which will probably shift to pre-eruptive re-pressurized phase, until the future eruption.

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: Eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

“…Prior to the 2014 Ontake eruption, a tiltmeter and broadband seismometer recorded rapid inflation of × 10 6 m at a depth of 1,000 m, which was likely caused by water boiling (Maeda et al 2017). The hydrothermal fluid associated with the 2014 Ontake eruption was derived from 3-6 km beneath the ground surface (Kato et al 2015;Narita et al 2019), which is 2-4 km deeper than for the 2018 MPCG eruption. Hydrothermal fluid from a deeper reservoir is likely to have a higher specific enthalpy due to its closer proximity to magma, leading more explosive eruption.…”

“…Assuming that most of the vapor condenses near the vent and releases latent heat to the plume, Qv is calculated as the product of the total vapor mass Mv (in kg) and the specific enthalpy Hv (kJ/kg) (Kagiyama et al 1981;Terada and Sudo 2012;Narita et al 2019):…”

“…Moreover, the volcanic ash cloud was monitored by weather radar (Meteorological Research Institute 2018). The volcanic plume track allows the heat discharge rate of an ash cloud to be estimated, which can then be converted to a mass flux of water (Briggs 1969;Kagiyama 1981;Terada and Sudo 2012;Narita et al 2019), although surveillance cameras and photographs taken from the ground did not constrain the height of the ash cloud at the climax of the eruption.…”

Coeruptive and posteruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption based on PALSAR-2 time-series analysis

“…The increased posteruptive subsidence on the southwestern side of the 2018 crater can be interpreted as the depressurization of the hydrothermal system, mass discharge and thermoelastic compaction (e.g., McTigue 1986;Narita and Murakami 2018). In the 2014 Ontake eruption, a plume from the vent was observed for more than two years after the eruption, where the magnitude of posteruptive deformation was at least 30 cm for three years (Narita et al 2019). For the 2018 Kusatsu-Shirane eruption, steam ejection from the 2018 crater has not been observed since February 22, 2018 and high-frequency earthquakes with volume changes had almost ceased by May 2018 (JMA 2018).…”

Coeruptive and posteruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption based on PALSAR-2 time-series analysis

“…The increased posteruptive subsidence on the southwestern side of the 2018 crater can be interpreted as the depressurization of the hydrothermal system, mass discharge and thermoelastic compaction (e.g., McTigue 1986; Narita and Murakami 2018). In the 2014 Ontake eruption, a plume from the vent was observed for more than two years after the eruption, where the magnitude of posteruptive deformation was at least 30 cm for three years (Narita et al 2019). For the 2018 Kusatsu-Shirane eruption, steam ejection from the 2018 crater has not been observed since February 22, 2018 and high-frequency earthquakes with volume changes had almost ceased by May 2018 (JMA 2018).…”