11:00 〜 13:00
[SVC28-P10] Temporal change in volcanic glow prior to vulcanian eruptions at Showa Crater of Sakurajima volcano, Japan
キーワード：ブルカノ式噴火、火映、桜島火山
Volcanic glow is a phenomenon that the plume above the crater glows red during nighttime due to the scattering of red light radiated from a red-hot area at the crater bottom. Because the presence of volcanic glow means the exposure of high-temperature zone such as magma, its brightness and spectra have information of high-temperature zone that is usually hidden beneath the cooled and solidified lava plug. Temporal change in brightness of volcanic glow has been reported and correlated with the dynamics of underlying magma. For example, Johnson et al. (2014) used the red color intensity of glow as a proxy of gas emission flux from lava dome at Santiaguito volcano (Guatemala) and found it correlates with the ground inflation-deflation cycles. In this study, we analyzed 90 events that accompanied volcanic glow before the onset of the eruption. The intensity of the glow was quantified by R-value of the image. We also calculated a ratio of G-value to R-value (GR ratio) to examine relative temperature change based on color-ratio pyrometry by a color CCD camera. In this study, we report two kinds of temporal change in volcanic glow at Showa Crater of Sakurajima volcano (Japan). One is a short-term rapid change approximately at 1 s before the onset of vulcanian eruptions. Another is a long-term gradual change several minutes to several seconds prior to vulcanian eruptions.
The rapid change in volcanic glow is well correlated to infrasound change. Vulcanian explosions typically generate pulse-like infrasound, which consists of an impulsive compression phase and a subsequent rarefaction phase. Yokoo et al. (2009) found a slight pressure increase (a ‘preceding phase’) 0.5–0.7 s before the arrival of the impulsive compression phase at the vulcanian eruption on January 2, 2007. They explained the preceding phase by the swelling (deformation) of the lava plug prior to the explosion. In our dataset, the infrasound preceding phase was found in 64 of 90 eruptions. Here we found that 28 eruptions show rapid glow increase that synchronizes with the infrasound preceding phase. The GR ratio also increases with the glow change, indicating a significant temperature increase that is possibly caused by the generation of deep fracture on the lava plug. On the other hand, however, 36 eruptions do not show rapid glow increase despite they show infrasound preceding phase. An interesting finding is that the duration of the infrasound preceding phase tends to be longer when they accompany the rapid glow increase.
The long-term increase in volcanic glow was recognized in 73 events except for 17 events which shows unclear fluctuations before the eruption onset. Therefore, the long-term glow change may be an essential process for vulcanian eruptions. The duration of the long-term increase ranges from 3 s to 5.5 min (median 32 s). The GR ratio does not show significant change during long-term glow increase, indicating the absence of temperature increase. We suggest that the increase in shallow fractures of the lava plug and subsequent gas leakage (increase in scattering) both contribute to the long-term glow change. This process may correspond to gas leakage before vulcanian eruptions (Iguchi et al., 2008). The duration of the gas leakage was estimated to be approximately 0–5 min at Showa Crater (Yokoo et al., 2013), which is consistent with the long-term glow increase.
Analysis of the volcanic glow of 90 eruptions suggested that the long-term (~several minutes) gas leakage through shallow fractures, which do not show significant temperature change, is an essential process for vulcanian eruptions. While the rapid (~1s) generation of deep fractures, accompanying a significant temperature increase, is a necessary process only for 31% of vulcanian eruptions. The latter observation may indicate that some vulcanian eruptions start from the generation of deep fracture at the lava plug, while others start from the deeper part beneath the lava plug.
The rapid change in volcanic glow is well correlated to infrasound change. Vulcanian explosions typically generate pulse-like infrasound, which consists of an impulsive compression phase and a subsequent rarefaction phase. Yokoo et al. (2009) found a slight pressure increase (a ‘preceding phase’) 0.5–0.7 s before the arrival of the impulsive compression phase at the vulcanian eruption on January 2, 2007. They explained the preceding phase by the swelling (deformation) of the lava plug prior to the explosion. In our dataset, the infrasound preceding phase was found in 64 of 90 eruptions. Here we found that 28 eruptions show rapid glow increase that synchronizes with the infrasound preceding phase. The GR ratio also increases with the glow change, indicating a significant temperature increase that is possibly caused by the generation of deep fracture on the lava plug. On the other hand, however, 36 eruptions do not show rapid glow increase despite they show infrasound preceding phase. An interesting finding is that the duration of the infrasound preceding phase tends to be longer when they accompany the rapid glow increase.
The long-term increase in volcanic glow was recognized in 73 events except for 17 events which shows unclear fluctuations before the eruption onset. Therefore, the long-term glow change may be an essential process for vulcanian eruptions. The duration of the long-term increase ranges from 3 s to 5.5 min (median 32 s). The GR ratio does not show significant change during long-term glow increase, indicating the absence of temperature increase. We suggest that the increase in shallow fractures of the lava plug and subsequent gas leakage (increase in scattering) both contribute to the long-term glow change. This process may correspond to gas leakage before vulcanian eruptions (Iguchi et al., 2008). The duration of the gas leakage was estimated to be approximately 0–5 min at Showa Crater (Yokoo et al., 2013), which is consistent with the long-term glow increase.
Analysis of the volcanic glow of 90 eruptions suggested that the long-term (~several minutes) gas leakage through shallow fractures, which do not show significant temperature change, is an essential process for vulcanian eruptions. While the rapid (~1s) generation of deep fractures, accompanying a significant temperature increase, is a necessary process only for 31% of vulcanian eruptions. The latter observation may indicate that some vulcanian eruptions start from the generation of deep fracture at the lava plug, while others start from the deeper part beneath the lava plug.