Massive eruption sent waves round world
Rumblings began on Dec. 18, 2021, under the small South Pacific islands of Hunga Tonga and Hunga Ha'apai in the Kingdom of Tonga, and continued for several weeks. On Jan. 15, an underwater volcano exploded upwards with a force not seen since 1883 and at 10 times the magnitude of Mount St. Helens.
An umbrella cloud developed at approximately 30 km above sea level, with a much higher central transient “overshoot.”
The two Hunga islands had been formed by ancient eruptions, and after one in 2015, ash and magma settled to fill the space between the two. But after January's violent explosion, the newer central part submerged, leaving the uninhabited Hunga Ha'apai and Hunga Tonga separate again.
The main island of Tonga was devastated by the ensuing tsunami.
None of the above is atypical of a large volcanic event, but thanks to advances over the decades, ground-based and space-borne instruments allowed researchers to observe in greater detail an eruption's unseen results.
Lamb waves are pressure waves of atmospheric fluid that result from volcanic eruptions and nuclear tests, and can last from minutes to several hours.
With January's eruption, scientists observed that these seismoacoustic waves circled the planet in one direction four times and back again three times — mirroring that of Indonesia's 1883 Krakatau eruption. Equally strikingly, a wave also travelled at between roughly 550 kps and 1,600 kps to an altitude of about 450 km.
“This atmospheric-waves event was unprecedented in the modern geophysical record,” lead author Robin Matoza, an associate professor at University of California Santa Barbara's department of Earth Science, said in a release.
Matoza led a team of 76 scientists from 17 countries to study the atmospheric waves.
Nine hours after the explosion, booms were heard as far off as Alaska — 10,000 kilometres away — compelling some to believe there was a link to the eruption.
But the scientists believe those booms could not have originated in Hunga. “I heard the sounds,” said co-author David Fee at the University of Alaska Fairbanks Geophysical Institute in the release, “but at the time definitely did not think it was from a volcanic eruption in the South Pacific.”
“While there's still much to learn, it's clear that standard sound models cannot explain how audible sounds propagated over such extreme distances. We interpreted that they were generated somewhere along the path by non-linear effects,” Matoza said.
“We have more than a century of advances in instrumentation technology and global sensor density,” he said. “So the 2022 Hunga event provided an unparalleled global data set for an explosion event of this size.”