4:30 PM - 4:45 PM
[SVC29-11] Characteristics of eruption periodicity observed at Shikabe geyser and estimation of the factor of long term periodicity change
Keywords:geyser, eruption periodicity change
The eruption periodicity of a geyser is not always regular but sometimes shows bimodal distribution or acts chaotically. Also, the eruption periodicity changes from short to long time scales. For example, the eruption periodicity at Old Faithful Geyser in Yellowstone National park was regular distribution in the 1800s, but gradually became bimodal distribution (Rinehart, 1969). Hurwitz et al. (2008) suggested that some geysers in Yellowstone National park may change their eruption periodicity due to the river discharge and precipitation. However, the mechanism of periodicity change has been discussed mainly statistically, so the physical parameters (hydraulic head, for example) controlling the eruption periodicity are not understood quantitatively.
In this study, we implemented observation at Shikabe geyser in southern Hokkaido to understand the mechanism of periodicity change of geyser, from November 26th, 2019 to February 24th, 2020 (first half) and from July 8th, 2020 to November 4th, 2020 (latter half). The intermittent cycle of the Shikabe geyser consists of the quiescence period—hot spring water does not discharge from the vent, the overflowing period—hot spring water overflows from the vent, and the eruption period. We measured the periodicity as the interval between the time of onset of overflow (IBO: Interval Between Overflow) from the temperature change of the Pt resistance thermometer attached at the vent. Then, we classified the periodicity into the quiescence time, overflowing time, and eruption time based on the acoustic pressure data which reflect onset and termination of eruption well, and investigated temporal change and the relative relationship of these times.
In the first half, IBO extended from approximately 12.5 min to 14.3 min, and the extending rate gradually decreased. We can find two distinctive modes of the periodicity: the stable mode distributing around the mean IBO, and the unstable mode fluctuating several minutes from the mean IBO. In particular, the unstable mode has a series of patterns in which once a long eruption occurs, the following quiescence time and the overflowing time are certainly extended, and the next quiescence time will be shortened. In the latter half, unlike the first half, the mean IBO decreased to approximately 11 min and changed to a double cycle. The quiescence time, overflowing time, and eruption time in the double cycle exhibit a pendulum-like behavior alternating back and forth around the mean IBO.
To understand the factor of the increase in mean IBO in the first half, we investigated the temporal change of the quiescence time, overflowing time and eruption time and found that the IBO extension is affected by the increase in both the quiescence time and overflowing time. Therefore, we made a hypothesis that the decrease in supply rate of the hot spring water to the geyser reservoir; the decrease in the hydraulic head causes the decrease in mean IBO in the first half. We formulated the IBO change mathematically using the equation of water ascent in the well based on Darcy’s law and calculated the hydraulic head change that explains the observed IBO change. As a result of the calculation, the IBO change in the first half can be explained by a decrease in the hydraulic head of about 0.7m. In the previous study, the pressure change in the aquifer was pointed out as one of the factors of periodicity change (Hurwitz et al., 2008; Kiryukhin, 2016). The result of this study is quite significant in that the amount of change in aquifer pressure can be estimated from the model equation of the IBO.
The decrease in aquifer pressure suggested from the long-term IBO change may be affected locally by the increase in pumping at the nearby hot springs, or regionally by the decrease in aquifer pressure in the entire hot spring area or the seasonal changes. However, we only have data for about a year and have no information on nearby hot springs, so we have not yet been able to specifically distinguish these factors. Therefore, we should not only continue observing for several years, but also obtain information about the use of nearby hot springs and changes in the amount of spring discharge.
In this study, we implemented observation at Shikabe geyser in southern Hokkaido to understand the mechanism of periodicity change of geyser, from November 26th, 2019 to February 24th, 2020 (first half) and from July 8th, 2020 to November 4th, 2020 (latter half). The intermittent cycle of the Shikabe geyser consists of the quiescence period—hot spring water does not discharge from the vent, the overflowing period—hot spring water overflows from the vent, and the eruption period. We measured the periodicity as the interval between the time of onset of overflow (IBO: Interval Between Overflow) from the temperature change of the Pt resistance thermometer attached at the vent. Then, we classified the periodicity into the quiescence time, overflowing time, and eruption time based on the acoustic pressure data which reflect onset and termination of eruption well, and investigated temporal change and the relative relationship of these times.
In the first half, IBO extended from approximately 12.5 min to 14.3 min, and the extending rate gradually decreased. We can find two distinctive modes of the periodicity: the stable mode distributing around the mean IBO, and the unstable mode fluctuating several minutes from the mean IBO. In particular, the unstable mode has a series of patterns in which once a long eruption occurs, the following quiescence time and the overflowing time are certainly extended, and the next quiescence time will be shortened. In the latter half, unlike the first half, the mean IBO decreased to approximately 11 min and changed to a double cycle. The quiescence time, overflowing time, and eruption time in the double cycle exhibit a pendulum-like behavior alternating back and forth around the mean IBO.
To understand the factor of the increase in mean IBO in the first half, we investigated the temporal change of the quiescence time, overflowing time and eruption time and found that the IBO extension is affected by the increase in both the quiescence time and overflowing time. Therefore, we made a hypothesis that the decrease in supply rate of the hot spring water to the geyser reservoir; the decrease in the hydraulic head causes the decrease in mean IBO in the first half. We formulated the IBO change mathematically using the equation of water ascent in the well based on Darcy’s law and calculated the hydraulic head change that explains the observed IBO change. As a result of the calculation, the IBO change in the first half can be explained by a decrease in the hydraulic head of about 0.7m. In the previous study, the pressure change in the aquifer was pointed out as one of the factors of periodicity change (Hurwitz et al., 2008; Kiryukhin, 2016). The result of this study is quite significant in that the amount of change in aquifer pressure can be estimated from the model equation of the IBO.
The decrease in aquifer pressure suggested from the long-term IBO change may be affected locally by the increase in pumping at the nearby hot springs, or regionally by the decrease in aquifer pressure in the entire hot spring area or the seasonal changes. However, we only have data for about a year and have no information on nearby hot springs, so we have not yet been able to specifically distinguish these factors. Therefore, we should not only continue observing for several years, but also obtain information about the use of nearby hot springs and changes in the amount of spring discharge.