Detection of underwater volcanic activity at stations belonging to the International Monitoring System - case of Izu Islands sequence

In October 2023, a sequence of shallow seismo-acoustic events occurred in Izu Islands archipelago, south of Honshu, Japan’s main island. The sequence started on October 1, and within a span of 8 days, approximately 70 events with magnitudes of 4.0 (International Data Centre mb) or higher were reported in the Reviewed Events Bulletin (REB) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). An unusual tsunami of up to 60 cm was reported on October 09 by the Japan Meteorological Agency. The authorities suggested that it was triggered by a relatively small magnitude earthquake, however, other related phenomena were also considered. An underwater volcano eruption or quake-triggered seabed landslide could have caused the tsunami. In this work, we show features of signals recorded at the hydrophone stations of the International Monitoring System (IMS), which indicate that the tsunami might have originated from volcanic activity. In addition, hydroacoustic signals were also observed at island seismic stations. This presentation demonstrates how IMS data may contribute to the identification of underwater events, such as volcanic activity, which may result in catastrophic phenomena. Even though the IMS was built to detect nuclear explosion tests, the data are available for civil applications and are distributed to tsunami warning centres.