2:45 PM - 3:00 PM
[SVC29-05] Structure of the shallow hydrothermal system in Hakone volcano inferred from InSAR analysis
Keywords:Hakone volcano, hydrothermal system, phreatic eruption, InSAR, surface displacement
During the 2015 unrest at Hakone volcano, satellite-based SAR observations captured surface deformation before, during, and after the phreatic eruption. Moreover, recent magnetotelluric surveys reveal the subsurface resistivity structure of the volcano. In this presentation, we introduce the surface displacement, which was considered to be related to activities of hydrothermal fluids, detected by InSAR analysis and these models. Furthermore, we will discuss the structure of the shallow hydrothermal system of Hakone volcano.
1. Surface Displacement at the Owakudani Fumarole Area during the 2015 Unrest
During the 2015 unrest, an abnormal blowout occurred at a steam production well (473 m deep) in Owakudani from May 3, and ALOS-2/PALSAR-2 repeatedly observed very local uplift (~200 m diameter) at Owakudani (Kobayashi et al., 2018; Doke et al. 2018). This surface displacement is explained by a Mogi source at an elevation of about 900 m (about 100 m deep). A magnetotelluric survey also confirms the existence of a small resistive body, which was interpreted as a vapor pocket locating almost at the Mogi source (Mannen et al., 2019).
2. Surface Displacement during the 2015 Phreatic Eruption and Intrusion of Fluid Crack
Observation of ALOS-2/PALSAR-2 before and after the phreatic eruption at Owakudani from June 29 to July 1, 2015, detected a surface displacement in an area about 1 km southeast of Owakudani (Doke et al., 2018). This displacement was explained by an NW-SE-trending crack opening and closing of a sill beneath the crack. The estimated crack location corresponds to a pre-existing fissure vent, suggesting that the crack was opened using the pre-existing structure. Moreover, a crater, which was considered the eruption center of a phreatic eruption that occurred in the past (Kobayashi et al., 2006; Kobayashi, 2008), is located on the estimated crack, and the opening of the crack is thought to have caused repeated phreatic eruptions. In the central cone of Hakone volcano, several crater chains with NW-SE strikes are distributed, suggesting the existence of similar cracks beneath the crater chains.
3. Surface Displacement during 2006-2011 by ALOS/PALSAR
The InSAR time series analysis (SBAS method) of ALOS/PALSAR showed steady subsidence on the west side of Owakudani (Doke et al., 2020), with a subsidence rate of 25 mm/yr estimated from the results of 2.5D analysis. This subsidence is explained by the contraction of a Mogi source at an elevation of 700 m (depth of 300 m), with about 10,000 m3/yr. The existence of such contraction suggests that there is a similar vapor pocket to the 2015 unrest, although they differ in depth and behavior.
These results suggest that the hydrothermal system in the shallow part of Hakone volcano is characterized by plural NW-SE to WNW-ESE striking crack-like fluid supply paths and pocket-like fluid reservoirs distributed at depths of several hundred meters.
Acknowledgments
ALOS/PALSAR and ALOS-2/PALSAR-2 data were provided by JAXA via the Coordinating Committee for the Prediction of Volcanic Eruption as part of the project, “ALOS Domestic Demonstration on Disaster Management Application,” of the Volcano Working Group. The original ALOS/PALSAR data belong to JAXA and METI, and the original ALOS-2/PALSAR-2 data belong to JAXA.
Reference
Doke et al. (2018) Earth, Planets and Space, 70, 63.
Doke et al. (2020) Remote Sensing, 12, 2842.
Kobayashi (2008) Research report of the Kanagawa Prefectural Museum. Natural history, 13, 43-60.
Kobayashi et al. (2006) Bulletin of the Volcanological Society of Japan, 51, 245-256.
Kobayashi et al. (2018) Earth and Planetary Science Letters, 491, 244-254.
Mannen et al. (2019) Earth, Planets and Space, 71, 135.
1. Surface Displacement at the Owakudani Fumarole Area during the 2015 Unrest
During the 2015 unrest, an abnormal blowout occurred at a steam production well (473 m deep) in Owakudani from May 3, and ALOS-2/PALSAR-2 repeatedly observed very local uplift (~200 m diameter) at Owakudani (Kobayashi et al., 2018; Doke et al. 2018). This surface displacement is explained by a Mogi source at an elevation of about 900 m (about 100 m deep). A magnetotelluric survey also confirms the existence of a small resistive body, which was interpreted as a vapor pocket locating almost at the Mogi source (Mannen et al., 2019).
2. Surface Displacement during the 2015 Phreatic Eruption and Intrusion of Fluid Crack
Observation of ALOS-2/PALSAR-2 before and after the phreatic eruption at Owakudani from June 29 to July 1, 2015, detected a surface displacement in an area about 1 km southeast of Owakudani (Doke et al., 2018). This displacement was explained by an NW-SE-trending crack opening and closing of a sill beneath the crack. The estimated crack location corresponds to a pre-existing fissure vent, suggesting that the crack was opened using the pre-existing structure. Moreover, a crater, which was considered the eruption center of a phreatic eruption that occurred in the past (Kobayashi et al., 2006; Kobayashi, 2008), is located on the estimated crack, and the opening of the crack is thought to have caused repeated phreatic eruptions. In the central cone of Hakone volcano, several crater chains with NW-SE strikes are distributed, suggesting the existence of similar cracks beneath the crater chains.
3. Surface Displacement during 2006-2011 by ALOS/PALSAR
The InSAR time series analysis (SBAS method) of ALOS/PALSAR showed steady subsidence on the west side of Owakudani (Doke et al., 2020), with a subsidence rate of 25 mm/yr estimated from the results of 2.5D analysis. This subsidence is explained by the contraction of a Mogi source at an elevation of 700 m (depth of 300 m), with about 10,000 m3/yr. The existence of such contraction suggests that there is a similar vapor pocket to the 2015 unrest, although they differ in depth and behavior.
These results suggest that the hydrothermal system in the shallow part of Hakone volcano is characterized by plural NW-SE to WNW-ESE striking crack-like fluid supply paths and pocket-like fluid reservoirs distributed at depths of several hundred meters.
Acknowledgments
ALOS/PALSAR and ALOS-2/PALSAR-2 data were provided by JAXA via the Coordinating Committee for the Prediction of Volcanic Eruption as part of the project, “ALOS Domestic Demonstration on Disaster Management Application,” of the Volcano Working Group. The original ALOS/PALSAR data belong to JAXA and METI, and the original ALOS-2/PALSAR-2 data belong to JAXA.
Reference
Doke et al. (2018) Earth, Planets and Space, 70, 63.
Doke et al. (2020) Remote Sensing, 12, 2842.
Kobayashi (2008) Research report of the Kanagawa Prefectural Museum. Natural history, 13, 43-60.
Kobayashi et al. (2006) Bulletin of the Volcanological Society of Japan, 51, 245-256.
Kobayashi et al. (2018) Earth and Planetary Science Letters, 491, 244-254.
Mannen et al. (2019) Earth, Planets and Space, 71, 135.