Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano

Matsunaga, Yasuo; Kanda, Wataru; Takakura, Shinichi; Koyama, Takao; Saito, Zenshiro; Seki, Kaori; Suzuki, Atsushi; Kishita, Takahiro; Kinoshita, Yusuke; Ogawa, Yasuo

doi:10.1016/j.jvolgeores.2019.106742

“…We also used the volume change of the point source explaining the cumulative eruptive deformation for approximately two years since the eruption. Although we should consider those factors to infer the ratio r v accurately, the inferred oblate deformation source does not contradict the alternating (thin) laminae of the low resistivity observed by the previous magnetotelluric survey (e.g., Matsunaga et al 2020).…”

Section: Data and Model Interpretationsmentioning

confidence: 64%

“…The best-t width of the plane in Model C is 1500 m, which indicates that the bottom of the plane reaches an approximately depth of 1200 m. In contrast, the best-t point source was inferred to be emplaced at the middle part of the eastern edge of the dislocation plane ( Figure 12). Previous magnetotelluric surveys proposed a low-resistivity subsurface structure from Moto-Shirane volcano to Yugama crater lake at 1500-3000 m below the surface, with alternating thin laminae of low-and highresistivity layers between the surface and large conductor (Nurhasan et al 2006;Matsunaga et al 2020).…”

Section: Data and Model Interpretationsmentioning

confidence: 99%

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

Himematsu

¹

,

Aoki

²

,

Ozawa

³

2020

Preprint

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Coeruptive deformation helps to interpret physical processes associated with volcanic eruptions. Because phreatic eruptions cause small, localized coeruptive deformation, we sometimes fail to identify plausible deformation signals. Satellite synthetic aperture radar (SAR) data allow us to identify extensive deformation fields with high spatial resolutions. Herein, we report coeruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption detected by time series analyses of L-band satellite SAR (ALOS-2/PALSAR-2) data. Cumulative deformation maps derived from SAR time series analyses show that subsidence and eastward displacement dominate the southwestern side of an eruptive crater with a spatial extent of approximately 2 km in diameter. Although we were unable to identify any significant deformation signals before the 2018 eruption, posteruptive deformation on the southwestern side of the crater has been ongoing until the end of 2019. This prolonged deformation implies the progression of posteruptive physical processes within a confined hydrothermal system, such as volcanic fluid discharge, similar to the processes observed during the 2014 Ontake eruption. Although accumulated snow and dense vegetation hinder the detection of deformation signals on Kusatsu-Shirane volcano using conventional InSAR data, L-band SAR with various temporal baselines allowed us to successfully extract both coeruptive and posteruptive deformation signals. The extracted cumulative deformation is well explained by a combination of normal faulting with a left-lateral slip component along a southwest-dipping fault plane and an isotropic deflation. Based on the geological background in which the shallow hydrothermal system develops across Kusatsu-Shirane volcano, the inferred dislocation plane can be considered as a degassing pathway from the shallow hydrothermal system to the surface due to the phreatic eruption. We reconfirmed that SAR data is a robust tool for detecting coeruptive and posteruptive deformations, which are helpful for understanding shallow physical processes associated with phreatic eruptions at active volcanoes.

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“…The number of volcanic earthquakes significantly decreases below sea level. Audio-magnetotelluric (AMT) (Nurhasan et al 2006) and magnetotelluric (MT) surveys (Matsunaga et al, 2020; Tseng et al, under review) detect a conductive region at a depth of 1-3.5 km from the surface, which almost corresponds low seismicity region in Fig. 2a.…”

Section: Seismicity and Hydrothermal System Structures At Kusatsu-shimentioning

confidence: 99%

“…However, the comparison between our result and hypocenter distributions significantly suggests that excitations of examined volcanic tremor relate to the seismogenic zone beneath Ainomine. According to AMT and MT soundings, the eastern flank of Kusatsu-Shirane is covered by a conductive layer with a thickness of 0.3-1 km (Nurhasan et al 2006;Matsunaga et al, 2020;Tseng et al, under review).…”

Section: Interpretation For Examined Volcanic Tremor and Eruption Promentioning

confidence: 99%

Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor amplitude

Yamada

¹

,

Kurokawa

²

,

Terada

³

et al. 2020

Preprint

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Kusatsu-Shirane volcano has been a particular study field for hydrothermal system and phreatic eruptions with plenty of thermal springs, fumaroles, and a crater lake of Yugama. On 23 January 2018, a phreatic eruption occurred at the Motoshirane cone of Kusatsu-Shirane, where no considerable volcanic activity had been reported in observational and historical records. To understand the eruption process of such a unique event, we examined observed seismic, tilt, and infrasound records. The onset of surface activity accompanied by infrasound signal was preceded by volcanic tremor and inflation of the volcano for 2 minutes. Tremor signals with a frequency of 5–20 Hz remarkably coincide with the rapid inflation. We apply an amplitude source location method to seismic signals in the 5–20 Hz band to estimate tremor source locations. Our analysis locates tremor sources at 1 km north of Motoshirane and at a depth of 0.5–1 km from the surface. Inferred source locations correspond a conductive layer of impermeable cap-rock estimated by magnetotelluric investigations, and an upper portion of the seismogenic region, suggesting hydrothermal activity hosted beneath the cap-rock. Examined seismic signals in the 5–20 Hz band are typically excited by volcano-tectonic events with faulting mechanism. Based on the above characteristics and background, we interpret that excitation of examined volcanic tremor reflects small shear fractures induced by sudden hydrothermal fluid injection to the cap-rock layer. The horizontal distance of 1 km between inferred tremor sources and Motoshirane implies lateral migration of the hydrothermal fluid, although we have not obtained direct evidence. Kusatsu-Shirane has a series of unrest at the Yugama lake since 2014. However, inferred tremor source locations do not overwrap active seismicity beneath Yugama. Therefore, our result suggests that the 2018 eruption was triggered by hydrothermal fluid injection through an independent pathway that has driven unrest activities at Yugama.

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“…The best-t width of the plane in Model C is 1500 m, which indicates that the bottom of the plane reaches an approximately depth of 1200 m. In contrast, the best-t point source was inferred to be emplaced at the middle part of the eastern edge of the dislocation plane ( Figure 12). Previous magnetotelluric surveys proposed a low-resistivity subsurface structure from Moto-Shirane volcano to Yugama crater lake at 1500-3000 m below the surface, with alternating thin laminae of low-and high-resistivity layers between the surface and large conductor (Nurhasan et al 2006;Matsunaga et al 2020). The large conductor implies the emplacement of volcanic uids that originated from the deep magma source and are con ned in a thick impermeable layer.…”

Section: Data and Model Interpretationsmentioning

confidence: 99%

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

Himematsu

¹

,

Ozawa

²

,

Aoki

³

2020

Preprint

0

View full text Add to dashboard Cite

Coeruptive deformation helps to interpret physical processes associated with volcanic eruptions. Because phreatic eruptions cause small, localized coeruptive deformation, we sometimes fail to identify plausible deformation signals. Satellite synthetic aperture radar (SAR) data allow us to identify extensive deformation fields with high spatial resolutions. Herein, we report coeruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption detected by time series analyses of L-band satellite SAR (ALOS-2/PALSAR-2) data. Cumulative deformation maps derived from SAR time series analyses show that subsidence and eastward displacement dominate the southwestern side of an eruptive crater with a spatial extent of approximately 2 km in diameter. Although we were unable to identify any significant deformation signals before the 2018 eruption, posteruptive deformation on the southwestern side of the crater has been ongoing until the end of 2019. This prolonged deformation implies the progression of posteruptive physical processes within a confined hydrothermal system, such as volcanic fluid discharge, similar to the processes observed during the 2014 Ontake eruption. Although accumulated snow and dense vegetation hinder the detection of deformation signals on Kusatsu-Shirane volcano using conventional InSAR data, L-band SAR with various temporal baselines allowed us to successfully extract both coeruptive and posteruptive deformation signals. The extracted cumulative deformation is well explained by a combination of normal faulting with a left-lateral slip component along a southwest-dipping fault plane and an isotropic deflation. Based on the geological background in which the shallow hydrothermal system develops across Kusatsu-Shirane volcano, the inferred dislocation plane can be considered as a degassing pathway from the shallow hydrothermal system to the surface due to the phreatic eruption. We reconfirmed that SAR data is a robust tool for detecting coeruptive and posteruptive deformations, which are helpful for understanding shallow physical processes associated with phreatic eruptions at active volcanoes.

show abstract

Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano

Cited by 32 publications

References 38 publications

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

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

Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor amplitude

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

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