Lessons from the Tonga Undersea Volcanic Eruption for the Maritime and Volcanic Nation of Japan

The eruption of the undersea volcano near Tonga reminded the world of disasters resulting from volcanic tsunamis.
Japan, which has many submarine volcanoes and volcanic islands, must strengthen its monitoring of these volcanoes and conduct world-leading offshore exploration and observation in preparation for catastrophic disasters caused by super-eruptions, which will definitely occur in the future.

The Tonga Undersea Volcanic Eruption

On January 15, 2022, a major eruption occurred at the Hunga Tonga–Hunga Ha'apai volcano, located in the Kingdom of Tonga in the South Pacific. The volcano has been active since 2021. Its plume rose to over 20 km, and explosions were said to have been heard as far away as New Zealand, over 2,000 km away. Although the details of the eruption, including its mechanisms, remain unclear, it was an event that shocked the world. Tsunamis were observed as far away as Japan and the Pacific Rim, including North America and Central and South America.
In Japan, volcanic disasters are not a topic of great concern, as they have fortunately been less frequent than earthquakes. However, our country is home to 111 active volcanoes (which have been active in the past 10,000 years), or 7% of the world's active volcanoes, making it the world's most volcanically active country. Moreover, as a maritime nation surrounded by the sea, about one-third of Japan's active volcanoes are distributed in the sea area.

Volcanic Tsunami Disasters

Most people know that undersea crustal alterations can cause tsunamis. These events are typically associated with ocean trench earthquakes, such as the Great East Japan Earthquake in 2011. However, volcanic tsunamis have also caused much damage in the past. In 1792, one of these disasters (the “Shimabara taihen, Higo meiwaku” (the Unzen earthquake and tsunami)) occurred around the Shimabara Peninsula, killing 15,000 people, the largest number of victims in Japan's history of volcanic disasters. The tsunami that followed the eruptions of Mount Komagatake in Hokkaido in 1640 and Mount Nishi on Oshima-Oshima Island in 1741 had maximum run-up heights of more than 20 meters. These disasters killed more than 700 and nearly 1,500 people, respectively. They were caused by volcanic activity that collapsed the volcanos’ structures and caused debris to flow into the sea (Figure 1(a)). By comparison, the recent eruption at the Hunga Tonga–Hunga Ha'apai in Tonga did not show any large-scale sector collapses.
When volcanic smoke rises on a large-scale, its balance of gravity is eventually upset, causing the eruption column to collapse. If the pyroclastic flow generated plunges into the sea, it might trigger a tsunami (Figure 1(b)). The eruption of large amounts of magma could also form a caldera on the seafloor, and tsunamis could be triggered by seafloor movement with subsidence of the seafloor (Figure 1(c)). A tsunami triggered by a large-scale undersea eruption also occurred 7,300 years ago after the super-eruption of the Kikai submarine caldera that devastated the Jomon culture in southern Kyushu. Waves in the vicinity exceeded 20 meters in height, and tsunamis with a height of several meters reached the coasts of present-day Oita, Kochi, and Wakayama. However, no pyroclastic flows or caldera collapses have yet been confirmed due to the recent Tongan eruption.
The widespread tsunamis occurred due to the Tongan event were most likely generated by the shock waves caused by the eruption, which excited the waves as they propagated and pushed against the sea surface, causing them to resonate and superimpose (Figure 1(d)).

Figure 1. The Primary Generation Mechanisms of Volcanic Tsunamis

The Urgent Need for Submarine Volcanoes and Volcanic Islands Exploration and Observations

The Tongan volcanic archipelago was formed when the Pacific Plate sank from Tonga-Kermadec Trench to below the Australian Plate. A chain of volcanoes extends from the land of New Zealand’s North Island through the Kermadec Islands to the Tonga Islands. In fact, its topography is extremely similar to that of the Japanese archipelago, especially the volcanic archipelago extending south from the Izu Peninsula (Figure 2). Firstly, the same Pacific plate is subducting at about the same rate as Japan. It is also built on top of the oceanic crust, which covers most of the Earth's seafloor. There are many undersea volcanoes in Japan, such as Nishinoshima Volcano, which is still active, and the Fukutoku Oka-no-Ba, which produced a flow of pumice stone last year, making the recent eruption and disaster in Tonga food for thought. Strengthening the monitoring of such submarine volcanoes and volcanic islands, including the occurrence of volcanic tsunamis as described above, is an urgent matter for Japan.
Since the recent eruption in Tonga, the domestic media has focused on submarine volcanoes and reported on their dangers. These events should not be surprising in Japan, which has large numbers of undersea volcanoes. With 111 volcanoes, our country is the most volcanic in the world. Therefore, we are constantly exposed to the danger of volcanic eruptions from both land and sea, including an eruption of Mount Fuji, which is often said to be imminent. However, there are also other matters we need to address. Namely, the risk of a super-eruption of a magnitude entirely incomparable to the event in Tonga. Looking only at the Holocene, the most recent geologic age (which started about 120,000 years ago), such super-eruptions have occurred at least 11 times at the seven volcanoes shown in Figure 2. Statistically, an eruption like this has a 1% chance of occurring in the next 100 years. The damage caused by an eruption like this would be catastrophic. Assuming a worst-case of super-eruption occurred in Kyushu, the island would be covered by hot pyroclastic flows, and more than 10 cm of volcanic ash would fall over Shikoku and the entire Honshu region. If this were to happen, all lifelines would cease to operate. Since restoration and rescue activities would be impossible, more than 100 million people would find themselves critically affected.
However, catastrophes of this magnitude are so infrequent that people are not even aware of them as natural disasters. Despite this, their hazard level (= number of expected deaths x annual probability of occurrence) is two orders of magnitude greater than that of a volcanic eruption of Mount Fuji, one order of magnitude greater than that of a torrential rain disaster or an earthquake directly under the capital, being comparable to that of a traffic accident. Therefore, it seems the mission of our generation is to come up with ways to mitigate the damage caused by such catastrophic volcanic disasters to ensure the survival of Japan as a nation and the Japanese as an ethnic group.
In addition, it is difficult to predict super-eruptions such as these in the present circumstances. This difficulty is caused mostly by a lack of accurate visualizations of the magma pools that lie beneath volcanoes, which are produced using the principles of CT scanning. This lack of progress is because it is nearly impossible to generate the artificial earthquakes required at multiple points on land with its dense populations.
However, exploration and observation of submarine volcanoes and volcanic islands, such as the Kikai submarine caldera,¹ which recently erupted in a super-eruption, can be conducted using ships. Kobe University has initiated a world-first attempt to visualize these magma pools with high precision. As a maritime and technological powerhouse, Japan should lead the world in focusing on mitigating the catastrophic volcanic disasters that are certain to occur in the future. Furthermore, to achieve constant, high-precision monitoring of active submarine volcanoes, it is also essential to conduct observational research using optical fiber networks installed on the seafloor. (End)

■Figure 2: Active volcanoes in Japan (triangles) and volcanoes that have caused super-eruptions (pink triangles).