May 16, 2022 8:58 am

Investigation underway for rare tsunami after Tonga eruption

Yurima Celdran 6 min
volcano tonga
Eruption of the Honga Tonga volcano seen from the Himawari satellite.

The great eruption of the submarine volcano Hunga Tonga, in the South Pacific, unleashed a meteotsunami on the eastern coast of Japan. Despite the fact that this country has been hit by tsunamis years ago, the Japan Meteorological Agency (JMA) states that nothing similar has ever been observed before. The meteotsunami arrived two hours earlier than expected and the waves reached a height higher than expected. What were the causes of this surprising phenomenon?

The arrival of the tsunami in Japan

The underwater volcano Hunga Tonga it erupted on the 15th at one o’clock in the afternoon (Japan local time). Six hours after the violent eruption, after seven in the afternoon, the JMA announced that there could be fluctuations in sea level, but without major consequences for the country’s coastal areas. Only an hour later, changes in sea level suddenly began to be observed, and before midnight it reached the Japanese shores a wave over a meter high.

The tsunami early warning systems, put into operation when the arrival of waves of around a meter in height are expected, were not activated in time because the systems did not detect this phenomenon. The Japan Meteorological Agency explained that this ‘Mysterious’ tsunami reacted in very different ways compared to tsunamis triggered by ordinary earthquakes and that, due to this, the measuring devices could not detect it in time.

its peculiarities

This exceptional tsunami had two unusual features compared to usual tsunamis. One of them was the little time that the wave needed to propagate and the other reach distant coastal areas.

Although the speed at which a tsunami caused by an earthquake spreads depends mainly on the average depth of the sea, it is almost always the same and is between 800 km/h offshore and 25 km/h on the coast. According to these data, and taking into account the distance that Japan was from the eruption zone (about 8000 km), the tsunami should have taken about 10 hours to reach its shores. However, this tsunami arrived about two hours earlier than expected.

The other great feature of this tsunami was the increase in wave height with distance. Generally, the tsunami caused by an earthquake is highest on the coast closest to the place of occurrence. However, this time, the tide level changed from 10 cm to 30 cm in the Micronesian islands, near Tonga, but reached about 1 meter in those of Japan.

So what are the possible reasons for each? Experts are paying attention to the fact that sudden changes in atmospheric pressure were observed near Japan.

Sudden changes in atmospheric pressure in Japan

The eruption was so explosive that it caused strong vibrations in the air creating atmospheric waves. About 7 hours after the eruption, these waves reached Japan and raised their atmospheric pressure about 2 hPa in 30 minutes. This sudden change in pressure pushed sea ​​level down about 2 cm before and after the first wave was observed of the tsunami. For these reasons, scientists believe that this tsunami could have reached the Japanese coast before because it was caused by atmospheric waves, and not by an earthquake as a result of the volcanic eruption, as suspected at the start of the study.

On the other hand, the elevation in the height of the wave near the coast of Japan was also associated with the atmospheric waves caused by the eruption. As experts observed after an exhaustive study, 80 years after the 1883 eruption of Krakatota, in Indonesia, they proposed a new theory: when such an explosive eruption occurs, sea ​​surface resonance causes the tsunami wave height to rise with distance.

Despite all the new discoveries, lTsunamis associated with volcanic activity are rare and extremely difficult to predict., so there is no special monitoring system. As a result of the new events and to avoid “unexpected” situations, it will be necessary to consider a monitoring and information transmission mechanism that captures the events as soon as possible.

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