Tonga eruption must serve as reminder of tsunami’s dangers

The cardinal rule for evacuating from tsunami is to flee if a tremor is felt. However, tsunami can come even if there is no tremor. It is hoped that people will pay attention to the diverse range of natural disasters and renew their awareness of the threat they pose.

A massive underwater volcanic eruption occurred off the South Pacific island nation of Tonga at about 1 p.m. on Saturday. The Japan Meteorological Agency initially predicted that the eruption would not have a significant impact on distant Japan.

However, tsunami reached various locations in Japan on Saturday night. The agency hastily issued tsunami warnings for areas including the Amami Islands in Kagoshima Prefecture, and tsunami advisories for the entire Pacific coast. A rise in atmospheric pressure was also observed across the nation.

Railway services along the coast were canceled. The Common Test for University Admissions was also canceled at a venue in Miyako, Iwate Prefecture. In Kochi Prefecture, many fishing boats were overturned.

Tsunami were also observed in North America and South America. In Tonga, flooding and disruptions in the communications system have hindered the country’s efforts to ascertain the situation, raising concerns about damage from the disaster.

The change in atmospheric pressure appears to have been caused by shock waves from the eruption. Experts said the shift in atmospheric pressure may have caused sea levels to change and triggered the tsunami. As such a phenomenon is rare, a detailed clarification of the mechanism is awaited.

There have been cases in which tsunami triggered by earthquakes in South America caused damage in Japan. However, because the latest tsunami did not result from the quake-triggered movement of a seabed, it would have been difficult to predict the tsunami.

Tsunami mostly occur following a major earthquake, so it makes sense to flee to higher ground as soon as a tremor is felt. However, there are exceptions to this rule, as eruptions, not earthquakes, have sometimes triggered tsunami.

In Indonesia, a volcano collapsed in 2018, sending a huge amount of sediment into the sea, and more than 400 people were killed by the resulting tsunami. Mt. Mayu in Shimabara, Nagasaki Prefecture, collapsed in 1792, triggering tsunami that struck Kumamoto Prefecture on the opposite shore, and 15,000 people are believed to have been killed.

The preconception that tsunami must be preceded by a strong tremor could hinder flexible responses to unexpected disasters.

The Earth’s enormous energy, involving the movement of the mantle inside the crust and other phenomena, is beyond human imagination. Last year, an unprecedented incident occurred in which a massive amount of pumice, ejected from an underwater volcano on the Ogasawara Islands, drifted ashore to coastal areas of Okinawa and other prefectures.

As massive eruptions of underwater volcanoes do not occur frequently, research in the field has not progressed. It is crucial to deepen our knowledge about unknown phenomena and use this knowledge to improve forecasting and observation systems.

— The original Japanese article appeared in The Yomiuri Shimbun on Jan. 17, 2022.