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Tsunami in Japan when the Tonga volcano erupts… What is the identity of the unexpected tsunami?

An anomalous underwater volcanic eruption

At 1:10 pm (Korean time) on the 15th of this month, a volcano with an unprecedented scale is erupting on the seafloor near Tonga in the South Pacific. Courtesy of NASA

At 1:10 pm (Korean time) on the 15th, an unprecedented volcanic eruption occurred on the seabed near Tonga in the South Pacific. When the Tonga volcano erupted, the sound of the eruption reached Alaska, USA. The cloud column created by volcanic ash and volcanic gas was 19.2 km long. According to data released by the United Nations Action Satellite Program (UNOSAT), a satellite image analysis organization of the United Nations, the area of ​​Yeouido in Seoul (about 2.85 million m²) has disappeared without a trace. The National Aeronautics and Space Administration (NASA) analyzed the volcanic eruption’s power to be about 10,000 kilotons based on TNT explosives. It is more than 600 times the size of the Hiroshima atomic bomb (15 to 16 kilotons). Experts believe that it is worth studying because it has a lot of unusual characteristics that are different from existing submarine volcanic eruptions as much as they are unusual in size.

○ The scale of the explosion is not large, but the destructive power is huge.

Satellite images taken before the eruption of the Tonga volcano in August 2020 (left) and on the 16th of this month, the day after the volcano erupted. Copernicus Sentinel-2 satellite imagery, the European Union’s Earth Observation Program. Provided by the French National Space Center (CNES)

The evaluation that this volcanic eruption is not the largest in history with a volcanic eruption index (VEI) of 4-5 or 5-6 is dominant. If the VEI is 6 or higher, volcanic ash and gas are ejected into the atmosphere, causing temporary climate change. The VEI of Volcano Pinatubo in the Philippines, which erupted in 1991, was 6. The eruption of Krakatau, Indonesia in 1883, VEI 6, wiped out two-thirds of the original island and dropped the global average temperature by 1.2 degrees over five years. In comparison, the eruption time of Volcano Tonga was short, around 10 minutes, and the amount of sulfur dioxide emitted was about 400,000 tons, which is not expected to affect the climate.

It is presumed that the Tonga Volcano has a large amount of gas in the magma and the eruption size has increased because the top of the volcano is close to sea level. Kim Seung-seop, a professor of geological and environmental sciences at Chungnam National University, said, “It is the magma property that determines the pattern of a volcanic eruption. To find out the exact cause and process of why this submarine volcano erupted, a later field investigation is needed.

○ A tsunami caused by the resonance phenomenon of atmospheric pressure change

Japan, which is located about 7900 km from the volcanic eruption point, announced that there was no possibility of a tsunami (tsunami) on the evening of the day of the eruption, but issued a tsunami warning at 00:00 on the 16th. At 10:52 PM on the 15th, a tsunami was already observed on Chichijima, Ogasawara Islands. The Japan Meteorological Agency predicted a tsunami height of up to 3 m, but in reality, a tsunami with a maximum height of 1.2 m was observed in Kominato, Amami Islands. Both the tsunami outbreak time and height predictions were wrong.

Japanese experts believe that this tsunami is completely different from other tsunamis. 80% of tsunamis are caused by tectonic motion, which causes the entire seawater to sway up and down. A series of tsunamis caused by landslides from volcanic eruptions account for 10% of cases. However, this tsunami is a meteorological tsunami caused by a sudden change in atmospheric pressure like a typhoon at the time of a volcanic eruption.

A tsunami occurs when the speed at which the atmospheric pressure change moves, that is, the speed at which the pressure trough of a typhoon moves is equal to the wave speed, resonance occurs and energy is amplified. Because it propagates through the air, it can have far-reaching effects much faster than earthquakes and tsunamis that travel through the ocean. However, tsunami is a phenomenon that occurs in the upper layer of the sea, so it is a rare phenomenon, accounting for only 3 to 4% of the total tsunami.

○ Possibility of simultaneous occurrence of tsunami and volcanic landslide and tsunami

Contradictory interpretations have also emerged. Professor Kim said, “If you look at the fact that part of the island disappeared after the volcanic eruption, it may have been a tsunami caused by a landslide at the time of the eruption.

In general, tsunamis occur in one direction because tsunamis are caused by resonance. However, it was analyzed that the tsunami occurred in all directions, centering on the volcano. Professor Kim said, “The first tsunami that arrived earlier than the Japan Meteorological Agency could not predict may be a tsunami, and the subsequent tsunami may be caused by a landslide. In Tonga, a detailed investigation into the volcanic eruption has not been carried out yet.

Lee Jung-ah, reporter for Donga Science [email protected]

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