JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC45] Active Volcanism

convener:Yuta Maeda(Nagoya University), Takahiro Miwa(National research institute for earth science and disaster prevention), Takeshi Nishimura(Department of Geophysics, Graduate School of Science, Tohoku University)

[SVC45-33] Mechanisms of infrasonic pulses accompanying the 2018 small phreatic eruption at Iwo-yama volcano, Kirishima Volcanic Complex, Japan

*Dan Muramatsu1, Takeshi Matsushima2, Mie Ichihara3 (1.Dept. Earth Planet. Sci, Graduate school of Science, Kyushu Univ., 2.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu Univ., 3.Earthquake Research Institute, Univ. of Tokyo)

Iwo-yama volcano, part of the Kirishima Volcanic Complex, which is located in southern part of Kyushu Island produced a small phreatic eruption on 19 April 2018. We analyzed infrasonic signals accompanied the eruption. Spectral analysis of the signal shows that the dominant frequency clearly lowered and low-frequency content of the signal significantly intensified about 4 hours after the onset of the eruption, and at the same time, a monitoring camera of JMA confirmed expansion of the vent area. At the time of the dominant frequency change, many high-frequency pulse-like signals were observed. On the other hand, low-frequency N-shape pulses were recorded continuously every ~2 seconds after the frequency change. We named the former as ‘A-type pulse’ and the later as ‘B-type pulse’. A-type pulse was found to be consisted of a low frequency part with a mean dominant frequency of 1.2 Hz and a high frequency part with a mean dominant frequency of 9.0 Hz. B-type pulse has a mean dominant frequency of 1.2 Hz, as same as the low-frequency part of A-type pulse. According to physical models of generation of infrasonic pulse, we interpret that the high-frequency part of A-type pulse is a bursting sound of bubble (or slag) rising in viscous mud, and the low-frequency part of A-type pulse and B-type pulse are produced by bubble vibration in a fluid. It might be possible that vent expansion and viscous mud formation occurred by weakening of medium around the vent and addition of hot water, which resulted in the generation of A-type pulse. Then, a mud-water suspension is formed by adding more water and shifted to the activity of B-type pulse. These results may provide important insights to understand dynamics and mechanisms of phreatic eruption.