口永良部島火山で発生するモノクロマティック微動 --2019年9月以降の古岳山体周辺における活動--

Abstract

Monochromatic events (MNCs), also known as N-type, T-type, or LP events, are volcanic earthquakes observed at plenty of active volcanoes. The characteristic waveform feature of sinusoidal oscillations is often interpreted as the MNC activity related to volcanic fluid supply at shallow depths. To understand the relation with volcanic activity, we investigate MNCs at Kuchinoerabujima volcano in the South of Kyusyu island, Japan. Focusing on the activity from September 2019, we find two major MNC activities in 2020 and 2022 regarding the seismic energy of MNCs (Emnc). The 2020 activity almost corresponds to an eruptive period at the Shindake cone. However, we find that most MNCs in the focused period occurred beneath the Furudake cone, about 1 km apart from Shindake. The 2022 activity is more intense with Emnc and quality factor Q. Most events in the 2022 activity are hybrid events with superposing high-frequency signals on the sinusoidal oscillations, which suggests that the fluid supply and oscillations trigger small fractures in the vicinity of the MNC source. We interpret the 2022 activity as representing the fluid supply that may have triggered particular seismicity, continuous tremor amplitude increase, and new crater lake formation at Furudake crater from July 2023.