Kaitoku Seamount is a three-peaked submarine volcano located about 1 km S of Tokyo, Japan. The base of the seamount has a diameter of 40 km and a height of 2.5 km above the ocean floor. The two peaks in the south are called Tokai Tokuba and Nishi Tokuba (figure 2). The most recent previous confirmed eruption was in 1984, when discolored water, gas-and-steam plumes, and floating pumice was reported (SEAN 09:10). Water discoloration was observed in December 1984 and June 1986, and the Japan Coast Guard (JCG) described activity in July 2001 which mainly consisted of bubbles. This report covers a new eruption that was characterized by strongly discolored water during August 2022 through January 2023 based on information from the Japan Meteorological Agency (JMA), Japan Coast Guard (JCG), and satellite data.

Figure 2. Submarine bathymetry map of Kaitoku Seamount. The island on the left is Nishi Tokuba (at a depth of 100 m) and the island on the right is Tokai Tokuba (depth of 97 m). The image date was not provided. Courtesy of JCG.

The JCG received a report on 19 August that discolored water was observed near the volcano on 18 and 19 August. This was also visible in a Sentinel-2 satellite image (figure 3). According to aerial observations by the JCG on 23 and 28 August, both discolored water and floating material were observed. The discolored water was yellow-white, approximately 100 m in diameter, and located near the Tokai Tokuba peak. On 28 August multiple zones of white discolored water were observed directly above the volcano and floating material covered an area approximately 200-m-wide and 6-km-long to the SE (figure 4).

Figure 3. Sentinel-2 satellite image showing a small amount of discolored water (bright blue-white) above Kaitoku Seamount on 19 August 2022. Image with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 4. Multiple zones of white discolored water above Kaitoku Seamount were visible during aerial observations on 28 August 2022. The floating material (yellow dotted circle) was identified to the SE in an area approximately 200 m wide and 6 km long. Courtesy of JMA (monthly report of activity at Kaitoku Seamount, August 2022).

Discolored water observations continued during September and October, which was interpreted as ongoing volcanic activity. Sentinel-2 satellite images showed small blue-white discolored plumes on 3, 8, 23, and 28 September that drifted SE, S, SW, and N from a single point. On 16 September an aerial observation conducted by JCG showed blue-white discolored water approximately 20 m wide and 30 m long (figure 5). Strong yellow-blue discolored water was visible on 28 September that mainly occurred around a single point, but a faint plume extended to the N. Though weather clouds sometimes prevented clear views of the vent area, discolored water plumes appeared more frequently in October. On 12 October there was a zone of dense yellow-white discolored water approximately 200 m in diameter; no floating material was observed (figure 6). Sentinel-2 satellite images captured discolored plumes on 3, 8, 13, 18, and 23 October, that mainly encompassed a single area. Part of the plumes extended SW, NW, E, and NE (figure 7).

Figure 5. Blue-white discolored water was observed during an aerial overflight of Kaitoku Seamount that was approximately 20 m wide and 30 m long at 1422 on 16 September 2022. Photo has been color corrected. Courtesy of JMA (monthly report of activity at Kaitoku Seamount, September 2022).

Figure 6. Aerial observation showing blue-white to green-yellow discolored water approximately 200 m in diameter at Kaitoku Seamount on 12 October 2022. Photo has been color corrected. Courtesy of JMA (monthly report of activity at Kaitoku Seamount, October 2022).

Figure 7. Bright blue-white to green-yellow discolored plumes at Kaitoku Seamount were visible in Sentinel-2 satellite images on 18 (left) and 23 (right) October 2022 that both drifted E and NE, respectively. The image taken on 18 October possibly captured material at the surface (darker circle). The white-and-gray irregular circles are atmospheric clouds, mostly seen in the 23 October image. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During November 2022 through January 2023 occasional strongly discolored plumes persisted. Sentinel-2 satellite images captured green-yellow discolored plumes on 22 and 27 November that drifted several kilometers SW and SE (figure 8). An aerial observation conducted by JCG on 25 November showed a yellow-white circular shape of discolored water approximately 300 m in diameter (figure 9). Additionally, blue-white discolored water was observed extending 1 km SW from the volcano and was about 10 km in length. Strong green-yellow discolored plumes were visible in Sentinel-2 satellite images on 17 and 27 December that drifted SE and S as far as 10 km from the vent area (figure 10). Frequent strongly discolored plumes were captured in satellite imagery on 1, 6, 11, 16, and 26 January 2023 that drifted in different directions for several kilometers. On 1 and 16 January white bubbles were also visible around the vent area (figure 11).

Figure 8. Sentinel-2 satellite images showing two strong green-yellow discolored plumes at Kaitoku Seamount on 22 (left) and 27 (right) November 2022. Some darker material may have also been captured at the sea surface accompanying the discoloration. The plume on 22 November drifted several kilometers SW and on 27 November a strong plume drifted SE for several tens of kilometers. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 9. A bright plume of yellow-white discolored water approximately 300 m in diameter at Kaitoku Seamount at 1357 on 25 November 2022 was observed during an aerial overflight. Photo has been color corrected. Courtesy of JMA (monthly report of activity at Kaitoku Seamount, November 2022).

Figure 10. Green-yellow discolored water plumes captured in Sentinel-2 satellite images at Kaitoku Seamount on 17 (left) and 27 (right) December 2022 drifted SE and S, and extended as far as 10 km from the vent area. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 11. Sentinel-2 satellite imagery showing green-yellow discolored water plumes drifting W and S from the vent area of Kaitoku Seamount on 1 (left) and 16 (right) January 2023. White bubbles were also visible above the vent area on both days. Images with “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Geological Summary. Submarine eruption have been observed during March 1984 and August 2022-January 2023 from Kaitoku Seamount (Kaitoku Kaizan), a three-peaked submarine volcano 80 km NNW of Kita-Ioto. The type of activity related to other observations of discolored water or bubbling, including an observation from 1543 CE, is uncertain.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/kaiikiDB/kaiyo20-2.htm); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).