15:00 〜 15:15
[SVC31-16] 上空二酸化硫黄量観測網による桜島の二酸化硫黄放出率：爆発的噴火前の放出率について
キーワード：二酸化硫黄放出率、桜島、爆発的噴火
Sulfur dioxide flux from an active volcano is an indicator for a magmatic involvement and is an important target for volcanic activity monitoring. The flux is estimated by multiplying a total cross-sectional SO2 amount of a volcanic plume and plume speed. An UV spectrometer instrument, which measures SO2 column amount of its field of view, is used for the observation and two main techniques (traverse and scanning) are usually used to obtain the cross-sectional SO2 amount. On Sakurajima volcano, Japan, there is a zenith SO2 amount measuring network consisting of 31 UV spectrometer instruments located along the surrounding road of the island from South to East in counterclockwise direction. When volcanic plume is flowing over the network, zenith SO2 column amount distribution along the network can be obtained every 10 seconds without making traverses beneath the plume, and the cross-sectional SO2 amount can be estimated using the distribution. Thus, SO2 flux monitoring with high-time resolution during the daytime is possible using the network. Similar kind of observation using SO2 observation array is carried out at Kilauea volcano and showed high quality SO2 flux monitoring (Elias et al., 2018).
Precursory inflations of volcanic edifice related to pressure increase at shallow part of the conduit are observed prior to Vulcanian eruptions (e.g., Iguchi et al., 2008). The pressure increase is explained by transition of the conduit from open to closed system due to formation of cap-rock or lava-plug. Some previous studies showed SO2 flux decrease prior to the explosive eruptions (e.g., Yokoo et al., 2013; Kazahaya et al., 2016) supporting the transition to closed system.
Using the network data between April and December 2018, I analyzed SO2 flux before the explosive eruptions of Sakurajima volcano. Since the network is working 9:00-16:30 and located only on South to East side of the volcano, SO2 flux variations related to 19 out of 246 explosive eruptions were observed by the network during the period. Ten out of nineteen explosive eruptions showed clear SO2 flux decrease prior to the eruptions which are similar to the previous studies. In contrast, eight eruptions did not show clear precursory decrease and one eruption showed precursory flux increase. Although some of the flux decreases may not have been detected due to short duration of the decrease (about < 5 min.), these observed SO2 flux variations may be suggesting diversity preparation process of the explosive eruptions.
Precursory inflations of volcanic edifice related to pressure increase at shallow part of the conduit are observed prior to Vulcanian eruptions (e.g., Iguchi et al., 2008). The pressure increase is explained by transition of the conduit from open to closed system due to formation of cap-rock or lava-plug. Some previous studies showed SO2 flux decrease prior to the explosive eruptions (e.g., Yokoo et al., 2013; Kazahaya et al., 2016) supporting the transition to closed system.
Using the network data between April and December 2018, I analyzed SO2 flux before the explosive eruptions of Sakurajima volcano. Since the network is working 9:00-16:30 and located only on South to East side of the volcano, SO2 flux variations related to 19 out of 246 explosive eruptions were observed by the network during the period. Ten out of nineteen explosive eruptions showed clear SO2 flux decrease prior to the eruptions which are similar to the previous studies. In contrast, eight eruptions did not show clear precursory decrease and one eruption showed precursory flux increase. Although some of the flux decreases may not have been detected due to short duration of the decrease (about < 5 min.), these observed SO2 flux variations may be suggesting diversity preparation process of the explosive eruptions.