Shallow Magmatic Hydrothermal Eruption in April 2018 on Ebinokogen Ioyama Volcano in Kirishima Volcano Group, Kyushu, Japan

Abstract

The Kirishima Volcano Group is a volcanic field ideal for studying the mechanism of steam-driven eruptions because many eruptions of this type occurred in the historical era and geophysical observation networks have been installed in this volcano. We made regular geothermal observations to understand the hydrothermal activity in Ebinokogen Ioyama Volcano. Geothermal activity resumed around the Ioyama from December 2015. A steam blowout occurred in April 2017, and a hydrothermal eruption occurred in April 2018. Geothermal activity had gradually increased before these events, suggesting intrusion of the magmatic component fluids in the hydrothermal system under the volcano. The April 2018 eruption was a magmatic hydrothermal eruption caused by the injection of magmatic fluids into a very-shallow hydrothermal system as a bottom–up fluid pressurization, although juvenile materials were not identifiable. Additionally, the upwelling of mixed magma–meteoric fluids to the surface as a kick was observed just before the eruption to cause the top–down flashing of April 2018. A series of events was generated in the shallower hydrothermal regime consisting of multiple systems divided by conductive caprock layers.

Reconstructing Surface Eruptive Sequence of 2018 Small Phreatic Eruption of Iwo-Yama Volcano, Kirishima Volcanic Complex, Japan by Infrasound Cross-correlation Analysis

“…We found two types of signals in the frequency range and their temporal uctuations and interpret this as the re ection 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 ows from W6 (Tajima et al 2019). Conversely, the signal from W6 exhibits a clearer correlation than that from W3 and W4 (Fig.…”

“…The vents W3 and W4 grew to a crater, around which ashy deposits and ejecta were observed. It is considered that the ejection occurred in the evening of April 20 (Tajima et al 2020). However, the eruption time in the West Crater area was not logged by the video recordings as it was nighttime.…”

Precursory ground deformation of the 2018 phreatic eruption on Iwo-Yama volcano, revealed by four-dimensional joint analysis of airborne and spaceborne InSAR

“…On March 19-21, 2017, mud pot F with a diameter of 1.8 m appeared. On April 15-18, 2017, a new fumarole vent H with vigorous fumarole discharge was formed, which is expressed as "jet fumarole" in Tajima et al (2020). On April 26, 2017, the mud emission created jet fumarolic vent A with a diameter of 1.5 m (Tajima et al 2020).…”

“…On April 15-18, 2017, a new fumarole vent H with vigorous fumarole discharge was formed, which is expressed as "jet fumarole" in Tajima et al (2020). On April 26, 2017, the mud emission created jet fumarolic vent A with a diameter of 1.5 m (Tajima et al 2020). These imply that increasing fluid injection to shallower levels promotes fracturing of the host medium at near-surface depths.…”

“…6), and the mud emission at jet fumarole vent A on April 26, 2017. Temperature measurements at 1-m depth within the geothermal area show that the geothermal anomaly area (GAA) with more than 50 °C extended to the south of peak A, close to fumarole vent H and mud pot F, from March 2017 (Tajima et al 2020). Deformation acceleration accompanied by intensified fumarole activity was also observed in Midagahara volcano during 2007-2010 where the acceleration of ground inflation was attributed to the increase in pore fluid pressure due to activated fumaroles overlapping on the most deformed area (Kobayashi 2018).…”