11:15 AM - 11:30 AM
[SVC33-08] Multi-site observations of SP and tilt at Onikobe Geyser, Miyagi Prefecture, Japan
Keywords:Geyser, Phreatic eruption, Tilt, Electric self-potential, Groundwater
[Introduction]
Several tens of minutes prior to the small phreatic eruption of Kirishima Iwo-Yama volcano in 2018, " A temporal change in tilt, tremor" and self-potential (SP) variations, interpreted as groundwater flow (streaming potential), were observed near the crater (Aizawa et al., 2022, Commun. Earth Environ.). Similarly, SP variations preceded geyser eruptions at Kirishima Iwo-Yama West Crater in 2021 (Tanabe et al., 2023, Earth Planets Space). These findings suggest similar groundwater flow processes before phreatic and geyser eruptions. To better understand the processes leading up to phreatic eruptions, it is crucial to investigate the relationship between groundwater flow and eruptions through geophysical observations, including SP measurements, at more accessible geyser sites. Tanabe et al. (2024, JpGU) conducted SP and tilt observations at Kibedani Geyser (CO2-driven, 21 °C), demonstrating their effectiveness in analyzing CO2-driven geysers. However, it remains unclear whether a similar mechanism applies to hydrothermal-driven geysers, which also provide insights into subsurface high-temperature fluids relevant to volcanic eruptions. This study reports results from multi-site SP and tilt observations at Onikobe Geyser, a hydrothermal-driven geyser in Miyagi Prefecture, Japan.
[Fields & Observations]
Onikobe Geyser (300 m a.s.l) erupts hydrothermal water (80–100°C) at 13.5-minute intervals, lasting 1.5 minutes. A cavity 3.5 m below the vent, crucial for eruptions, has been suggested (Teshima & Nishimura, 2024). From Oct 29 to Nov 1, 2024, we installed eight SP sensors, four tilt meters (two with acoustic sensors), thermometers, and cameras around the vent (Fig. 1). Temperatures at depths of 4.0–8.5 m inside the vent were also measured.
[Results]
Video and acoustic data indicate a single geyser cycle: Preplay (10–24 sec before eruption), Eruption (1 min 24 sec), and Recharge (12 min). SP data from six sites and tilt data from two sites recorded variations correlated with the geyser cycle. SP variations, referenced to the onset of the Preplay phase, are categorized into two phase:
Variation ①: SP increases during Preplay, peaks ~20 sec before eruption ends, and decreases until ~1 min post-eruption.
Variation ②: SP increases ~1 min post-eruption, decreases from ~4 min post-eruption until the next Preplay onset.
Tilt data show subsidence toward the vent from Preplay onset until ~20 sec before eruption ends, then uplift toward the vent until the next Preplay. Temperature measurements inside the conduit suggested 108-112°C during the eruption, decreased to 98°C ~1 minute after the eruption, stabilized for ~3 minutes, and then rapidly increased.
[Discussion & Conclusion]
SP variations are interpreted as groundwater flow (streaming potential). Assuming a negative zeta potential, the downstream side corresponds to high SP. In this study, groundwater flows toward the vent twice: during Variation ① and Variation ②. Variation ① suggests groundwater inflow toward the vent increases during subsidence (indicating depressurization) and starts decrease during uplift (indicating pressurization). Variation ② reflects groundwater inflow toward the vent ~1 minute after the eruption, decreasing ~4 minutes later. Inside the conduit, the temperature stabilizes at 98°C ~4 minutes after the eruption. We interpreted that volume reduction caused by the phase transition from vapor to liquid immediately after the eruption and around 1 minute after the eruption, groundwater begins to flow into the subsurface system. By ~4 minutes after the eruption, the volume of groundwater inflow starts to decrease due to the increased proportion of vapor-phase fluid in the subsurface system caused by heating, leading to the transition to the next Preplay phase.
[Acknowledgments]
We thank H. Oka and staffs at the Onikobe geyser. This work was supported by KAKENHI (grant number JP23H01275, JP23K25971).
Several tens of minutes prior to the small phreatic eruption of Kirishima Iwo-Yama volcano in 2018, " A temporal change in tilt, tremor" and self-potential (SP) variations, interpreted as groundwater flow (streaming potential), were observed near the crater (Aizawa et al., 2022, Commun. Earth Environ.). Similarly, SP variations preceded geyser eruptions at Kirishima Iwo-Yama West Crater in 2021 (Tanabe et al., 2023, Earth Planets Space). These findings suggest similar groundwater flow processes before phreatic and geyser eruptions. To better understand the processes leading up to phreatic eruptions, it is crucial to investigate the relationship between groundwater flow and eruptions through geophysical observations, including SP measurements, at more accessible geyser sites. Tanabe et al. (2024, JpGU) conducted SP and tilt observations at Kibedani Geyser (CO2-driven, 21 °C), demonstrating their effectiveness in analyzing CO2-driven geysers. However, it remains unclear whether a similar mechanism applies to hydrothermal-driven geysers, which also provide insights into subsurface high-temperature fluids relevant to volcanic eruptions. This study reports results from multi-site SP and tilt observations at Onikobe Geyser, a hydrothermal-driven geyser in Miyagi Prefecture, Japan.
[Fields & Observations]
Onikobe Geyser (300 m a.s.l) erupts hydrothermal water (80–100°C) at 13.5-minute intervals, lasting 1.5 minutes. A cavity 3.5 m below the vent, crucial for eruptions, has been suggested (Teshima & Nishimura, 2024). From Oct 29 to Nov 1, 2024, we installed eight SP sensors, four tilt meters (two with acoustic sensors), thermometers, and cameras around the vent (Fig. 1). Temperatures at depths of 4.0–8.5 m inside the vent were also measured.
[Results]
Video and acoustic data indicate a single geyser cycle: Preplay (10–24 sec before eruption), Eruption (1 min 24 sec), and Recharge (12 min). SP data from six sites and tilt data from two sites recorded variations correlated with the geyser cycle. SP variations, referenced to the onset of the Preplay phase, are categorized into two phase:
Variation ①: SP increases during Preplay, peaks ~20 sec before eruption ends, and decreases until ~1 min post-eruption.
Variation ②: SP increases ~1 min post-eruption, decreases from ~4 min post-eruption until the next Preplay onset.
Tilt data show subsidence toward the vent from Preplay onset until ~20 sec before eruption ends, then uplift toward the vent until the next Preplay. Temperature measurements inside the conduit suggested 108-112°C during the eruption, decreased to 98°C ~1 minute after the eruption, stabilized for ~3 minutes, and then rapidly increased.
[Discussion & Conclusion]
SP variations are interpreted as groundwater flow (streaming potential). Assuming a negative zeta potential, the downstream side corresponds to high SP. In this study, groundwater flows toward the vent twice: during Variation ① and Variation ②. Variation ① suggests groundwater inflow toward the vent increases during subsidence (indicating depressurization) and starts decrease during uplift (indicating pressurization). Variation ② reflects groundwater inflow toward the vent ~1 minute after the eruption, decreasing ~4 minutes later. Inside the conduit, the temperature stabilizes at 98°C ~4 minutes after the eruption. We interpreted that volume reduction caused by the phase transition from vapor to liquid immediately after the eruption and around 1 minute after the eruption, groundwater begins to flow into the subsurface system. By ~4 minutes after the eruption, the volume of groundwater inflow starts to decrease due to the increased proportion of vapor-phase fluid in the subsurface system caused by heating, leading to the transition to the next Preplay phase.
[Acknowledgments]
We thank H. Oka and staffs at the Onikobe geyser. This work was supported by KAKENHI (grant number JP23H01275, JP23K25971).