4 − 1. Camera (movie) data
Movies S1 and S2 show examples of intermittent hydrothermal discharge at W4. On the basis of the movie data for vent Fs, we divided the eruption cycle into four phases as follows: (i) cessation of steam effusion; (ii) onset of hydrothermal discharge; (iii) active steam effusion; and (iv) drain-back of the hydrothermal pool at vent Fs. Here, we explain each phase on the basis of visual camera observations. (i) Effusion of steam from vents Fs and Fn stopped approximately 20–40 min before the onset of hydrothermal discharge (Fig. 3b; Movies S1 and S2). (ii) Hydrothermal discharge started slowly without steam from vent Fs. Discharged water flowed into two parts: inflow into W4 and accumulation at vent Fs, which together formed a hydrothermal water pool (Fig. 3b; Movies S1 and S2). (iii) Steam effusion at vents Fs and Fn gradually became active, and hot water in the pool of vent Fs gradually decreased (Movie S2). (iv) Approximately 1–1.5 h before the next hydrothermal discharge, drain-back of the hydrothermal water pool occurred at vent Fs (Movie S2). The small water pool at vent Fs in some instances dried up before the timing of drain-back, in which case, drain-back of hydrothermal water could not be visually confirmed (Fig. 3).
4 − 2. Temperature data
Figure 3 shows the variation in temperature and SP during the period of multi-parametric observations. During strong steam effusion, the temperatures of vents Fs and Fn were maintained at ~ 96 ℃, which is the boiling point at this elevation (1233 m; Fig. 3). The temperature decreased at both vents when steam effusion stopped. Approximately 20–40 min after the cessation of steam effusion, hydrothermal discharge started, and the temperature at vent Fs increased rapidly (Fig. 3b). When hydrothermal discharge began, the temperature was 80–90 ℃ but increased to 96 ℃ 7–14 min later. The temperature at vent Fn started to increase approximately 12–20 min later than that at vent Fs, probably because the position of vent Fn was approximately 30 cm higher relative to vent Fs (Fig. S1), and thus the upwelling hydrothermal water filled vent Fn later than Fs.
4 − 3. Electric self-potential (SP) data
SP shows clear cyclic change (Fig. 3). Figure 3b exhibits a typical pattern of SP temporal change associated with surface hydrothermal discharge, whereas Fig. 3c shows the pattern without surface hydrothermal discharge. In the latter case, SP shows clear temporal change and is accompanied by a small change in temperature (0.2–0.9 ℃). For events with hydrothermal discharge, the temporal change in SP starts approximately 2 h before the onset of hydrothermal discharge (Fig. 3b). For events without hydrothermal discharge, the temporal change in SP starts 1 to 3 h before a slight temperature rise (Fig. 3c). It should be noted that there was no fumarolic zone or hydrothermal discharge at the locations of four (two radial, two tangential) electrodes (Fig. 2). Therefore, a thermoelectric effect can be excluded as a cause of variation in SP. Previous studies in New Zealand of Iodine geyser with an average cycle of 160–180 s and Pohutu geyser with a cycle of several minutes have shown that cyclic SP change can be associated with geyser eruptions (Nishi et al., 2000; Legaz et al., 2009). The temporal change in SP at geysers has been explained by an electrokinetic mechanism involving groundwater movement through porous material (e.g., Mizutani et al., 1976; Ishido and Mizutani, 1981). In a typical rock–water system, the downstream direction shows positive voltage (i.e., a positive SP zone represents the region to which groundwater flows). In this study, we similarly interpret a change in SP as being generated by subsurface groundwater flow. It is noted that the electrokinetic mechanism depends on the electric charge separation in an electrical double layer whose thickness is in the order of nanometers (e.g., Ishido and Mizutani, 1981; Revil et al., 1999). The surface area of the solid–water interface governs the amount of electric charge. Therefore, SP cannot be generated by groundwater flow in a pipe-like conduit or fractures owing to the insufficient electric charge in such structures but can be generated by groundwater flow in porous material. This is an important aspect to consider with respect to identifying the mechanism of intermittent hydrothermal discharge.
4–4. Seismic data
For seismic data, we divided signals into two frequency ranges: (A) signals in the frequency range of > 20 Hz, and (B) signals in the frequency range of < 20 Hz. Figure 4 shows an example of the multi-variable data for one cycle associated with the hydrothermal discharge. Figure 5 shows multi-variable data for four cycles, all of which show hydrothermal discharge. Signal (A) started to decrease when drain-back occurred (“iv” in Fig. 4), but signal (B) started to decrease prior to the occurrence of drain-back. Approximately 1 to 4 h after the onset of hydrothermal discharge (“iii” in Fig. 4), signal (B) increased rapidly (Figs. 4 and 5) but there was no corresponding change in signal (A). Given these differing patterns, seismic signals (A) and (B) are thought to have different tremor sources.
4–5. Acoustic data
For acoustic data, we focused on signals in the frequency range of > 20 Hz because infrasound (< 20 Hz) is highly affected by wind. The acoustic signals decreased when drain-back occurred (“iv” in Fig. 4). After the onset of hydrothermal discharge, the signals gradually increased (“ii” in Fig. 4). Approximately 1 to 4 h later, the signals increased more dramatically (“iii” in Figs. 4 and 5). We interpret that the acoustic signals with a frequency of > 20 Hz were generated from surface activity, such as splashing of the water surface of the hydrothermal pool at vent Fs. The acoustic signals and seismic signals with a frequency of > 20 Hz may have the same origin, as changes in both signal types occurred simultaneously (“iii” in Fig. 4).
4–6. Tilt data
Change in tilt associated with hydrothermal discharge was identified from the measured tilt data. Because tilt data can be highly affected by rainfall and temperature, we calculated the moving average using a 4 h window to remove short-term fluctuations. Figure 4 shows that E–W tilt (i.e., uplift in the direction toward the vent Fs) started to increase around the time of onset of change in SP. When hydrothermal discharge occurred, the E–W tilt showed subsidence in the direction toward the vent (“ii” in Fig. 4). When the seismic signals increased rapidly (“iii” in Fig. 4), the change in E–W tilt stopped. It is noted that such correlation is found in other cycles, but temporal change also occurred when no geophysical change was observed (Figs. 5 and S2). Because the cycle of the hydrothermal water discharge was long, in the range of 14–70 h, the tilt data may have been affected by unknown noise. However, the tilt data are consistent with the SP data, whereby uplift and subsidence of the vent Fs are consistent with the accumulation and discharge of groundwater, as suggested by SP data.
4–7. Data after the period of multi-parametric observations
After the period of multi-parametric observations (20 April to 4 May 2021), we continued camera and temperature observations, which overall covered the period 20 April to 20 September 2021 (Table 1). This period of extended observations captured a significant change in the style of hydrothermal discharge. Figure 6 shows temperature data measured at vents Fs and Fn, rainfall data recorded at the Ebino observation site (JMA; Fig. 2), and the timing of hydrothermal discharge. On the basis of these data, we defined three discharge styles, as follows.
Style 1 (Movie S2) involved hydrothermal discharge from vent Fs and steam effusion from vent Fn. This style was observed during the period of multi-parametric observations (Fig. 3d). The discharge interval was approximately 14–70 h. This style was mainly identified as occurring until 11 May 2021.
Style 2 (Movie S3) was observed after heavy rain from 14 May to 22 May and involved hydrothermal discharge simultaneously from vents Fs and Fc from 22 May to 6 June. The discharge interval was approximately 18–40 h.
Style 3 (Movie S4) was observed from 4 to 28 June. The upper panel in Fig. 6 shows the change in temperature and the timing of hydrothermal discharge, revealing repeated hydrothermal discharges from vent Fs with an interval of 1–2 h multiple times in a row. After multiple hydrothermal discharges, steam effusion continued for a few hours until the next multiple hydrothermal discharges occurred.