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[SVC31-07] Volcanic glow fluctuation prior to vulcanian eruptions at Showa Crater, Sakurajima volcano, Japan
Keywords:Vulcanian eruption, Volcanic glow, Sakurajima volcano
Introduction
A volcanic glow is occasionally observed at active volcanoes, which is interpreted that radiative visible light from red-hot bodies (e.g., magma) is reflected by fumarolic gas or clouds above the crater. At Showa Crater of Sakurajima volcano, the glow sometimes appears before vulcanian eruptions. Aizawa et al. (2013, JpGU) reported that the glow intensity increases from at ~1 s before the onset of the explosion and which synchronizes with the infrasound preceding phase (Yokoo et al., 2009). In this study, we re-analyze these glow events and discuss their implications for triggering processes of vulcanian eruptions.
Methods
We analyzed visible light video recordings (30 fps) and infrasound data observed at Kurokami station (Kyoto Univ.) for 36 events from 2011 to 2012. For video recordings, we calculated R-value averaged over a rectangular range (30×60 pixels) set above the crater for each frame to quantify glow intensity. For infrasound, we shifted the waveforms to their origin times based on the estimated travel times and then compared them with the time-series change of R-value.
Results
In addition to the correspondence of glow increase and infrasound preceding phase that reported by Aizawa et al. (2013, JpGU), we found 13 events in which a gradual increase in glow intensity began 2-33 seconds before the infrasound onset. The glow gradually intensifies from a few seconds to several tens of seconds before the eruption (we call stage Ⅰ), and then increases rapidly with the onset of infrasound preceding phase (stage Ⅱ), leading to an explosion.
Discussion
Since it is thought that the lava dome (caprock) caps the crater bottom before vulcanian eruptions, thermal radiations from the exposed high-temperature body (red-hot portion) due to crack formations on the dome surface may cause the glow. In this case, based on the theory of thermal radiation, there are two possible reasons for glow intensity increase: (a) temperature increase in the red-hot portion and (b) expansion of the red-hot portion without temperature increase. Thus, we calculated G/R ratio (green light / red light) based on a methodology of color ratio pyrometry as an indicator of temperature change. Consequently, we suggest that the glow intensity increased by the mechanism (b) in stage Ⅰ and (a) in stage Ⅱ. Therefore, stage Ⅰ may reflect, e.g., the widening of cracks due to the expansion of the lava dome and accompanying expansion of the red-hot portion, and stage Ⅱ correspond to a rapid temperature increase due to an opening of crack which reaches a gas pocket beneath the caprock.
A volcanic glow is occasionally observed at active volcanoes, which is interpreted that radiative visible light from red-hot bodies (e.g., magma) is reflected by fumarolic gas or clouds above the crater. At Showa Crater of Sakurajima volcano, the glow sometimes appears before vulcanian eruptions. Aizawa et al. (2013, JpGU) reported that the glow intensity increases from at ~1 s before the onset of the explosion and which synchronizes with the infrasound preceding phase (Yokoo et al., 2009). In this study, we re-analyze these glow events and discuss their implications for triggering processes of vulcanian eruptions.
Methods
We analyzed visible light video recordings (30 fps) and infrasound data observed at Kurokami station (Kyoto Univ.) for 36 events from 2011 to 2012. For video recordings, we calculated R-value averaged over a rectangular range (30×60 pixels) set above the crater for each frame to quantify glow intensity. For infrasound, we shifted the waveforms to their origin times based on the estimated travel times and then compared them with the time-series change of R-value.
Results
In addition to the correspondence of glow increase and infrasound preceding phase that reported by Aizawa et al. (2013, JpGU), we found 13 events in which a gradual increase in glow intensity began 2-33 seconds before the infrasound onset. The glow gradually intensifies from a few seconds to several tens of seconds before the eruption (we call stage Ⅰ), and then increases rapidly with the onset of infrasound preceding phase (stage Ⅱ), leading to an explosion.
Discussion
Since it is thought that the lava dome (caprock) caps the crater bottom before vulcanian eruptions, thermal radiations from the exposed high-temperature body (red-hot portion) due to crack formations on the dome surface may cause the glow. In this case, based on the theory of thermal radiation, there are two possible reasons for glow intensity increase: (a) temperature increase in the red-hot portion and (b) expansion of the red-hot portion without temperature increase. Thus, we calculated G/R ratio (green light / red light) based on a methodology of color ratio pyrometry as an indicator of temperature change. Consequently, we suggest that the glow intensity increased by the mechanism (b) in stage Ⅰ and (a) in stage Ⅱ. Therefore, stage Ⅰ may reflect, e.g., the widening of cracks due to the expansion of the lava dome and accompanying expansion of the red-hot portion, and stage Ⅱ correspond to a rapid temperature increase due to an opening of crack which reaches a gas pocket beneath the caprock.