5:15 PM - 6:45 PM
[SVC29-P03] Eruption off the coast of Okinahama, Ioto in 2023
Keywords:Ioto, phreatomagmatic eruption, submarine eruption, Okinahama, monochromatic volcanic tremor, infrasound
Ioto is a volcanic island in the Ogasawara Islands. Japan Meteorological Agency (JMA) continuously monitors Ioto volcanic activity. The ground temperature is high throughout the island, and there are many fumarolic areas. In recent years, the entire island has been uplifting at an unusually rapid rate, and eruptions have occurred in various parts of the island.
According to the Ioto Air Base Corps of the Japan Maritime Self-Defense Force (SDF), eruptions have been observed repeatedly off the coast (Okinahama coast) in the south of the island since 2021. Previous researches revealed these events were phreatomagmatic (Nagai et al., 2022; Teguri et al., 2022). The eruptive activity after 2023 can be divided into two periods, one is in June and the other is after October. We introduce the eruptions off-Okinahama in these two periods, on the basis of the information from the SDF and geophysical observation data of JMA.
In the first period, the eruptions off-Okinahama were observed between 15 and 25 in June, 2023. The intervals of each event were every few minutes to over ten minutes. The intensity (e.g. height of water columns by these submarine eruptions) of these events was small and was almost the same as that of the previous off-Okinahama events. During the period, a discolored water, floating pumices and monochromatic volcanic tremors (TP-type tremor) accompanied by each eruption were observed. Such tremors were observed during the previous off-Okinahama activities (Teguri et al., 2022). Infrasound waves followed by the eruptions were often recorded in this period. No significant crustal deformation was observed associated with these eruptions.
In the second period, the temporal variations of the characteristics of the volcanic activities (height of water columns, frequency of the eruptions, and waveforms of the tremors and the infrasounds) were observed after October 2023. These variations may indicate changes of eruption processes. The eruption interval was every few minutes in the early stage of eruptive activity, but on November 3 the interval became a few seconds, and from November 4 the style changed to eruptions every few minutes with explosions and infrasounds that could be felt physically. Thereafter, eruptions occurred intermittently with intervals of several tens of minutes. The eruption was accompanied by a number of intermittent TPs. At the observation station near the Okinahama, infrasounds associated with TPs were observed, and these amplitudes became larger and clearer from October 30.
Since that time, TPs have become smaller in their amplitudes and in number. On the other hand, seismic signals excited by infrasounds associated with the eruptions began to be observed and were frequent after November 3 and continued until November 9. Many infrasounds with the amplitudes exceeding 10 Pa were observed. As ejecta deposits above the sea floor became thick due to an eruptive activity, a depth of subaqueous eruption point became possibly shallower. We consider changes in amplitudes of TP and infrasound events, and the coupling between the infrasound and the ground motion indicate changes in depth of eruption points. In addition, a top of the deposit of ejecta appeared above sea level near the eruptive point on October 30. Following the eruptive activities, the terrestrial area grew until early December and after that, it decreased gradually due to erosion.
During this period, volcanic earthquakes slightly increased in November, but there was no increase of them associated with the eruption. In addition, the GNSS continuous observations show that the north-south baseline elongation temporarily accelerated in November, synchronizing with the increase in volcanic earthquakes. In late November, subsidence centering on Mt. Motoyama, in the central-northern part of the island, was also observed. We have no clear idea to explain the relationship between these phenomena and the eruptive activity.
According to the Ioto Air Base Corps of the Japan Maritime Self-Defense Force (SDF), eruptions have been observed repeatedly off the coast (Okinahama coast) in the south of the island since 2021. Previous researches revealed these events were phreatomagmatic (Nagai et al., 2022; Teguri et al., 2022). The eruptive activity after 2023 can be divided into two periods, one is in June and the other is after October. We introduce the eruptions off-Okinahama in these two periods, on the basis of the information from the SDF and geophysical observation data of JMA.
In the first period, the eruptions off-Okinahama were observed between 15 and 25 in June, 2023. The intervals of each event were every few minutes to over ten minutes. The intensity (e.g. height of water columns by these submarine eruptions) of these events was small and was almost the same as that of the previous off-Okinahama events. During the period, a discolored water, floating pumices and monochromatic volcanic tremors (TP-type tremor) accompanied by each eruption were observed. Such tremors were observed during the previous off-Okinahama activities (Teguri et al., 2022). Infrasound waves followed by the eruptions were often recorded in this period. No significant crustal deformation was observed associated with these eruptions.
In the second period, the temporal variations of the characteristics of the volcanic activities (height of water columns, frequency of the eruptions, and waveforms of the tremors and the infrasounds) were observed after October 2023. These variations may indicate changes of eruption processes. The eruption interval was every few minutes in the early stage of eruptive activity, but on November 3 the interval became a few seconds, and from November 4 the style changed to eruptions every few minutes with explosions and infrasounds that could be felt physically. Thereafter, eruptions occurred intermittently with intervals of several tens of minutes. The eruption was accompanied by a number of intermittent TPs. At the observation station near the Okinahama, infrasounds associated with TPs were observed, and these amplitudes became larger and clearer from October 30.
Since that time, TPs have become smaller in their amplitudes and in number. On the other hand, seismic signals excited by infrasounds associated with the eruptions began to be observed and were frequent after November 3 and continued until November 9. Many infrasounds with the amplitudes exceeding 10 Pa were observed. As ejecta deposits above the sea floor became thick due to an eruptive activity, a depth of subaqueous eruption point became possibly shallower. We consider changes in amplitudes of TP and infrasound events, and the coupling between the infrasound and the ground motion indicate changes in depth of eruption points. In addition, a top of the deposit of ejecta appeared above sea level near the eruptive point on October 30. Following the eruptive activities, the terrestrial area grew until early December and after that, it decreased gradually due to erosion.
During this period, volcanic earthquakes slightly increased in November, but there was no increase of them associated with the eruption. In addition, the GNSS continuous observations show that the north-south baseline elongation temporarily accelerated in November, synchronizing with the increase in volcanic earthquakes. In late November, subsidence centering on Mt. Motoyama, in the central-northern part of the island, was also observed. We have no clear idea to explain the relationship between these phenomena and the eruptive activity.