長周期音波が地球周回　トンガ海底火山の大噴火―東大地震研

　トンガ諸島付近の海底火山「フンガトンガ・フンガハアパイ火山」の大噴火で発生した音波の一種が地球を周回したことが分かったと、東京大地震研究所の西田究准教授が１７日発表した。世界各地にある微気圧計の観測データを調べた成果で、１９８０年代に精度の高い観測が始まって以来、初めてという。
　この音波は大噴火で発生した音波のうち、周期が約１０００秒と長く、非常に低い音で、人の耳には聞こえない。減衰せずに遠くまで届くが、西田准教授は「地球周回が観測されたのはそれだけ噴火が大きかったからだ」と説明した。（2022/01/17-19:09）

A type of a sound wave generated by the recent huge eruption of a submarine volcano near the Tonga islands has circled Earth, a Japanese researcher said Monday.
   Such a phenomenon was observed for the first time since highly-precise surveys began in 1980s, Kiwamu Nishida, associate professor at the Earthquake Research Institute of the University of Tokyo, said.
   Nishida said data collected from microbarometers across the world showed that the generated sound wave had a very long time period of some 1,000 seconds and was inaudible.
   A long-period sound wave can travel a long distance without a decline, but "the observed wave even went around the globe because the eruption was so powerful," he noted.
   According to Nishida, a sound wave generated by the 1883 Krakatau volcanic eruption in Indonesia is believed to have gone around the planet. Meanwhile, there have been no reported data indicating the generation of a similar sound wave from the eruption of Mount Pinatubo in the Philippines in 1991.