A water flow model of the active crater lake at Aso volcano, Japan: fluctuations of magmatic gas and groundwater fluxes from the underlying hydrothermal system

Abstract

The first crater of Nakadake, peak of Aso volcano, Japan, contains a hot water lake that shows interesting variations in water level and temperature. These variations were discovered by precise, continuous observations of the lake independent of precipitation. We developed a numerical model of a hot crater lake and compared with observational data for the period from July 2006 to January 2009. The numerical model revealed seasonal changes in mass flux (75-132 kg/s) and enthalpy (1,840-3,030 kJ/kg) for the fluid supplied to the lake. The relation between the enthalpy and mass flux indicates that the bottom input fluid is a mixture of high-and lowtemperature fluids. Assuming a mixture of hightemperature steam at 800°C and liquid water at 100°C, we evaluated the liquid and steam fluxes. The liquid water flux shows a seasonal increase lagging behind the rainy season by 2 months, suggesting that the liquid water is predominantly groundwater. The fluctuation pattern in the flux of the high-temperature steam shows a relation with the amplitude of volcanic tremor, suggesting that heating of the hydrothermal system drives the tremor. Consequently, precise observations of a hot crater lake represent a potential method of monitoring volcanic hydrothermal systems in the shallow parts of the volcanoes.

Variety and sustainability of volcanic lakes: Response to subaqueous thermal activity predicted by a numerical model

“…The occurrence of an impermeable layer may explain the existence of volcanic lakes. Indeed, magnetotelluric surveys [Kanda et al, 2008] detected a layer of hydrothermal alteration at 100À200 m beneath the floor of Yudamari, a hot crater lake at Aso volcano, Japan [Terada et al, 2012;Shinohara et al, 2015], which may act as an impermeable layer. However, it is plausible that past volcanic explosions repeatedly destroyed the crater floor, including the underlying impermeable alteration layer, and replaced it with fresh eruption material.…”

Volcanic smoke reduction in visible and thermal infrared imagery

“…Accordingly, it is likely that the apparent temperature of the lake surface in the right bottom side of the lake on the image (47-48 °C) is closest to the actual water temperature. Given that the IR thermometry gives systematically lower values than direct measurements from a buoy by 8.3 °C, which is empirically confirmed by Terada et al (2012), and assuming the skin effect (slight decrease in water temperature in a boundary layer at the very surface of the lake; Oppenheimer 1997) as 2 °C (Terada et al 2008), the real water temperature on that day is estimated to be 53-54 °C. Besides the zonal distribution, we recognize several hot spots in the central part of the lake.…”

“…Undulation due to the steam filaments mentioned above does not account for these hot spots, since it is confirmed that these spots are stable regardless of additional compositing. We consider that these hot spots correspond to the upwelling of high-temperature water or steam that has been envisaged based on the heat and water balance of Yudamari by Terada et al (2012). The image processing of the present study can contribute to the detection of such spot-like temperature anomalies also for other crater lakes.…”

“…Temperature of the crater lake provides important information in evaluating the thermal discharge rate from the lake surface (Terada et al 2012;Terada and Hashimoto 2017). Figure 7a shows a snapshot of the surface temperature of Yudamari lake that was taken by an TIR camera on February 2, 2009.…”

Temporal variation in the resistivity structure of the first Nakadake crater, Aso volcano, Japan, during the magmatic eruptions from November 2014 to May 2015, as inferred by the ACTIVE electromagnetic monitoring system

“…Terada et al (2012) showed that changes in the shallow hydrothermal system can be tracked by monitoring enthalpy and mass flux to the crater lake. It is also desirable to monitor subsurface electrical resistivity, which is sensitive to the pore fluid/magma conductivity (e.g., Hermance 1995;Gaillard 2004).…”