Monitoring Submarine Volcanic Activities in the Izu–Bonin–Mariana Arc Using the International Monitoring System Hydrophone Array Signals

Abstract International Monitoring System hydrophone arrays are powerful tools for monitoring submarine volcanic activity. In this study, we routinely analyzed the signals of the triplet H11S, located off the coast of Wake Island, beginning in July 2022, to monitor submarine volcanic activity, particularly in the Izu–Bonin–Mariana arc. We removed the instrument response and applied a band-pass filter with cutoff frequencies of 4 and 8 Hz. Subsequently, the hydrophone signals were divided into 10 s time windows without overlapping. Semblance analysis was performed to determine the incoming direction and apparent velocity. The maximum obtained semblance values typically ranged from 0.5 to 0.9, and only values ≥0.7 were interpreted as significant. In our observation period (July 2022 to August 2024), we detected significant hydroacoustic signals from six sites (Ahyi, Daikoku, Kaitoku, Sofugan, Sumisu-jima, and Hachijo-jima) and compared them with those obtained from the past eruptions at Fukutoku-Oka-no-Ba. We discussed the characteristics of individual waveforms and spectrograms of some detected signals and found that frequency index values, defined as the ratio of mean spectral amplitudes between higher and lower frequency components, could be helpful in objectively discriminating the origins of hydroacoustic signals.