Figure 3 shows the results of the cross-correlation analysis. The top panel (Fig. 3a) shows the waveforms filtered at 1–7 Hz and 7–20 Hz. The middle and bottom panels show t-τ plots of the cross-correlation coefficient in the frequency ranges of 1–7 Hz (Fig. 3b) and 7–20 Hz (Fig. 3c), both of which exhibit high correlation values around τ =1.5 s after 15:39:45 on April 19. This lag time is equivalent to that expected for infrasound from South Crater, which confirms that its eruptive activity continued over the analyzed period after the onset. The correlation values in the lower-frequency band increase at 19:19:40 (Fig. 3b), similar to the amplitude values (Fig. 3a). Simultaneously, CAM1 captured the widening of the Y2a vent, which began at 18:40:51, with increased ash plumes from the vent (Additional File 2: Fig. A1). We consider that an intense eruption, accompanying remarkable low-frequency infrasonic signals, began at 19:19:40 after the vent widening. From around 3:57 on April 20, the correlation values around τ =1.5 s in the higher-frequency band intensified again (Fig. 3c). Besides, subtle peaks in τ <0.9 s, indicating West Crater signals, appear around 21:00 on April 20 in the 7–20 Hz band; however, the correlation pattern is unclear to constrain the onset (Fig. 3c).
We improved the resolution using the following procedure: The data were resampled up to 500 Hz by linear interpolation to increase the lag time resolution. We then calculated the cross-correlation coefficients for 5 s-long time windows sliding in 2 s, and progressively stacked them every 20 s. The results for the higher-frequency band are shown in Fig. 4, and the results for the lower-frequency band are provided in Additional File 3.
By analyzing the South Crater signals, the lag time of the initial stage of the eruption is 1.55 s, whereas that after the reincrease (3:57 on April 20) is 1.50 s. (Fig. 4b). Referring to the expected lag times of the South Crater signals (Table A1), we consider that the shortening of the correlation peak lag time by 0.05 s indicates the source shift from Y2a to Y3. The lag time starts shortening simultaneously with the vent widening at 18:40:51, and before the low-frequency power increase at 19:19:40 (Fig. 3b). The images from CAM1 also show increased ash plumes from Y3 with the widening of Y2a (Fig. A1). We interpret that Y3 was activated with the vent widening and then, from around 3:57 on April 20, became the main source of the higher-frequency infrasonic signals.
At West Crater, there is no clear correlation in the corresponding range τ before 21:00 (Fig. 4c), although CAM2 recorded faint steam clouds from around 16:30. A possible reason is that the signal was below the detection level, or was buried in the strong signal from South Crater. At 21:05:20 ± 10 s, a correlation peak appears around τ =0.83 s (Fig. 4c), which corresponds to vent W6 (Additional File 1: Table A1). The correlation decays after 22:00. Another peak then appears around τ =0.72 s from 21:42:20 ± 10 s (Fig. 4c), which we infer as the onset of the small eruption from vents W3 and W4 that erupted ash and ejecta.
The relationships between the correlation peak time lags and the source positions are subject to uncertainties regarding the atmospheric temperature and the local wind speed. We checked the validity of the interpretations described above. For South Crater, the peak at τ =1.55 s is related to Y2a with the atmospheric sound speed and temperature of 336 m/s and 281 K, respectively. If the signal is from Y3, the atmosphere should have 326 m/s and 265 K, respectively. Alternatively, if the peak at τ =1.50 s is made by Y2a, it requires 347 m/s and 300 K. These values are outside the range of the estimated atmospheric temperature. It is also unrealistic that the temperature increased by nearly 20 K from 4 pm to 4 am; neither can we assume that the wind speed increased more than 10 m/s when considering the low-frequency noise level of the infrasound data (Fig. 3a). For West Crater, if the peak at τ =0.83 s corresponds to W3 and W4 instead of W6, the sound speed and the atmospheric temperature need to be as low as 292 m/s and 212 K, respectively. If the peak at τ =0.72 s is vent W6, it requires extremely large values of 390 m/s and 379 K. Neither the wind can generate such propagation speeds.
We reconstructed the sequence of the multiple-vent activities of the 2018 small phreatic eruption of Iwo-yama (Fig. 5). South Crater began an eruption at 15:39 on April 19, mainly from Y2a (and Y2b). The vent widened from 18:40 to 19:19, following which the eruption intensified, accompanying lower-frequency infrasonic signals. Vent Y3 was activated with the Y2a widening and became the main source of higher-frequency infrasonic signals from 3:57 on April 20. The steam-emitting vents appeared in the West Crater area around 16:30 on April 20, and the eruptions accompanying infrasonic signals occurred from 21:05 successively, in multiple vents. These observations suggest that the intense eruptions with remarkable infrasonic signals started with several hours of delay after the onset of steaming or the appearance of the vents.
Several hours of delay in the vent formation accompanying ash falls with intensified seismic tremor after the infrasound onset was also observed in the 2015 small phreatic eruption of the Hakone volcano, Japan (Yukutake et al. 2017). The delay has been interpreted as the time for the hydrothermal fluid to reach the ground surface from a subsurface open-crack source, while the infrasound began concurrently with the crack opening by strain transfer (Yukutake et al. 2018). The time delay between the onset of surface phenomena and the intense eruptions can be a common feature of small-scale phreatic eruptions. Our results add another case and will be useful for considering the mechanism of the transition process of small-scale phreatic eruptions and assessing the hazards of eruptions that form multiple vents.
We found two types of signals in the frequency range and their temporal fluctuations and interpret this as the reflection of the dynamics and transition of the eruptive activity. At the West Crater area, the geological survey found ash deposits and fragments around W3 and W4 (Tajima et al. 2020), but only traces of mud flows from W6 (Tajima et al. 2019). Conversely, the signal from W6 exhibits a clearer correlation than that from W3 and W4 (Fig. 4c), indicating that the former has more significant power. It is noticed that the infrasonic power does not necessarily represent the amount of ash emission. The generation mechanism of infrasound accompanying phreatic eruptions is only partially understood (e.g., Jolly et al. 2016), and we plan to investigate the source process of the infrasonic signals at Iwo-yama in future work.