Undersea volcanic eruption’s shock waves led to tsunami in Japan, expert says

The Yomiuri Shimbun
Fumihiko Imamura

Shock waves from an undersea volcanic eruption in the South Pacific Ocean likely triggered tsunami that struck Japan from Saturday night through Sunday, said tsunami engineering expert Fumihiko Imamura, a professor at Tohoku University.

The eruption occurred Saturday afternoon off the island nation of Tonga, generating a series of waves that culminated in tsunami recorded across areas of Japan’s Pacific coast, according to Imamura’s analysis.

In tsunami triggered when earthquakes create changes to the topography of the seafloor, the period of the surface waves usually ranges from 10 minutes to an hour, according to Imamura. In particular, when tsunami travel long distances, the period tends to be longer.

National Institute of Information and Communications Technology via AP
This combination of satellite images taken by Himawari-8, a weather satellite operated by the Japan Meteorological Agency, shows an undersea volcano eruption of the Pacific nation of Tonga on Saturday.

In contrast, the latest tsunami observed in Japan had periods of just a few minutes and moved up and down quickly.

Imamura said this phenomenon makes it unlikely the latest tsunami could have been triggered by changes to the seafloor following the eruption or by the impact generated when material spewing from the volcano fell into the ocean, the professor said.

According to the Japan Meteorological Agency, changes in atmospheric pressure were observed in the range of about 2 hectopascals in various parts of Japan from 8 p.m. to just past 9 p.m. Saturday.

By analyzing changes in tide levels and atmospheric pressure, Imamura found that tsunami were observed following rises in air pressure.

“The shock waves caused the changes in atmospheric pressure, traveling all the way to Japan, creating waves that move up and down in short periods,” Imamura said. “These waves are believed to have accumulated in waters near Japan to turn into large tsunami.”