Interpretation of detections of volcanic activity at Ioto Island obtained from in situ seismometers and remote hydrophones of the International Monitoring System

Abstract

In-situ seismic observations identified that volcanic activity of Ioto (formerly Iwojima), a volcanic island offshore Japan, increased in early September 2018. Observations of discolored nearshore waters and a splash reported by a local flyover provided evidence for a connection between undersea eruptions and recorded seismic activity. However there remain uncertainties as to when the undersea eruption series commenced and how much of the in-situ seismic activity recorded on the island was associated with volcanic earthquakes versus undersea eruptions. During this period, a large number of underwater acoustic (hydroacoustic) signals were recorded by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydroacoustic station HA11, at Wake Island (U.S. Territory), in the northwestern Pacific Ocean with signals with directions of arrival consistent with sources located at Ioto. The analysis presented here interprets signal features of the remote hydroacoustic recordings provided by HA11 in order to attempt to distinguish between volcanic earthquake signals and undersea eruption signals originating from Ioto. Histograms of hydroacoustic events interpreted as originating from Ioto correlate well with the in-situ seismic observations at Ioto in the early stage of volcanic activity. The results presented suggest that around 75% of the signals detected at HA11 with directions of arrival consistent with Ioto as their origin could be associated with undersea eruptions, supporting the conclusion that the IMS hydroacoustic stations can contribute to volcanic event remote monitoring.

Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at Ioto Island, Japan

“…TR2 did not accompany apparent infrasound signals. Matsumoto et al (2019) reported that remote hydrophones detected no relevant signal on September 12, either, even though small splashes were observed in shallow water near the Okinahama coast. Explosions under shallow water and violent water jets into the air generate detectable infrasound signals (Ichihara et al 2009;Lyons et al 2019Lyons et al , 2020, but those were not detected in this case.…”

“…The island, about 8 km × 4 km in size with the highest elevation of 170 m, is just the summit part of a stratovolcano rising about 2000 m from the sea floor. Eruptions sometimes occur under the sea and are detected by remote hydrophones (Matsumoto et al 2019). The seismic activity is intense and a large-scale uplift has continued for centuries (Kaizuka et al 1985;Ueda et al 2018).…”

“…It was preceded by predominant uplift and highfrequency seismicity for a few weeks. Data from local seismometers and remote hydrophones during the period indicates that frequent undersea eruptions associated with volcano-tectonic earthquakes occurred (Matsumoto et al 2019). From September 12 to 13, the number of long-period events and volcanic tremors increased while that of volcano-tectonic earthquakes decreased (Japan Meteorological Agency 2018).…”

“…At Ioto Island, phreatic eruptions frequently occur due to the high geothermal activity (Notsu et al 2005), and the volcanic activity is pronounced not only on the ground but also undersea detected by remote hydrophones (Matsumoto et al 2019). In this situation where the volcanic activity is very high throughout the island, there is a need to monitor eruptions and its temporal changes.…”

Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at an isolated volcanic island, Ioto, Japan

“…Ioto is an isolated volcanic island of which seismicity is regularly intense (Ueda et al 2018). At Ioto, phreatic eruptions frequently occur due to the high geothermal activity (Notsu et al 2005), and the volcanic activity is pronounced not only on the ground but also undersea detected by remote hydrophones (Matsumoto et al 2019). In this situation where the volcanic activity is very high throughout the island, there is a need to monitor eruptions and its temporal changes.…”