Multi-parametric observations of intermittent hydrothermal water discharge in West Crater of Iwo-Yama volcano, Kirishima Volcanic Complex, Japan

Abstract From April to July 2021, West Crater at Iwo-Yama, Kirishima Volcanic Complex, Japan, was repeatedly filled with hydrothermal water and subsequently evacuated. The overall cycle lasted 14–70 h, the course of a single followed this sequence phases: (i) steam effusion disappeared 20–40 min before discharge; (ii) discharge occurred, generating pool; (iii) resumed gradually increased; (iv) drain-back (evacuation) occurred 1–1.5 h onset next discharge. We used multi-parametric observations (optical camera, thermometer, electric self-potential (SP) electrodes, seismometer, acoustic sensor, tiltmeter) investigate cause cyclic A change in SP data approximately 2 However, small when did not occur. temporal is inferred have been caused by groundwater flow through region below Crater, implying that occurring polarity suggests flowed toward underlying vents. Seismic signals frequency range < 20 Hz decreased 15–45 after SP. This seismic signal pattern bubble activity boiling fluid. interpret inflow cold inhibited conduit, which turn cessation both activity. suggest Pressurization sufficient force upper part conduit ascend could built up lower owing decrease input into continuing supply bubbles hot water. increase pressure finally led surface. geyser key control on occurrence cyclicity Iwo-Yama. Graphical