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How Tonga’s volcanic eruption can help detect tsunamis

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On January 15, the cataclysmic eruption of a submerged volcano in the South Pacific devastated the archipelagic nation of Tonga, unleashed tsunamis around the world, and created a sonic boom heard as far away as Alaska. 

The blast was captured by a host of sensors located in land, sea, and sky. Two groups of scientists published their analyses of this data on May 12 in the journal Science

One team concluded that pressure waves from the event were comparable in scale to those from the massive 1883 Krakatau eruption in Indonesia, which unleashed clouds of ash that reached 50 miles high and explosions that could be heard 2,200 miles away at its peak. The second group explored how the pressure waves caused tsunamis to arrive on distant shores hours earlier than expected. This information can help scientists better understand the processes underlying eruptions and improve tsunami early-warning systems, the researchers say.

“There’s been nothing like this in the modern digital era,” says Robin Matoza, a geophysicist at the University of California, Santa Barbara and coauthor of one of the papers. “It’s a really remarkable event.”

Randy Cerveny, a meteorologist at Arizona State University and rapporteur on weather and climate extremes for the United Nations’s World Meteorological Organization, described the two papers as “fascinating work.”

“The more information we have now—and continued analyses of that available information—hopefully will make us better prepared for future eruptions of such incredible magnitude,” he said in an email.

The volcano responsible for all this turmoil lies 40 miles from Tongatapu, the largest island of Tonga. It’s roughly 12 miles across and topped by 3-mile-wide caldera with two “lips” that protrude above the water. The Hunga volcano experienced several minor eruptions from 2009 to 2015. A series of more violent outbursts began last December, climaxing with the massive January 15 eruption, which sent a cloud of ash more than 20 miles into the sky.

[Related: How to survive a tsunami]

Matoza’s team, which included researchers from 17 countries, investigated the pressure waves emitted by the powerful eruption. They compiled measurements from seismometers, buoy-based pressure sensors, hydrophones, weather satellites, instruments that record the bending of radio waves as they pass through the Earth’s atmosphere, and more.

The researchers focused in particular on Lamb waves, which are low-frequency perturbations that travel along the surface of the Earth at roughly the speed of sound. Cerveny likens the phenomenon to the behavior of a jiggly dessert.

“Think of an explosion from the bottom of a huge container of, say, Jell-O and how the pressure ripples of compressed jello would spread out horizontally from the explosion site,” he said.

Lamb waves typically span the full depth of the atmosphere, allowing researchers to track them with a variety of sensors both on the ground and in satellites, said Siddharth Krishnamoorthy, an aerospace engineer at the NASA Jet Propulsion Laboratory in Pasadena and coauthor of the paper. “The amplitude of the Lamb wave that we observed here was not so high as to cause damage, but it does help us understand wave propagation in the atmosphere and characteristics of the eruption itself,” he said in an email.

He and his collaborators observed that the Lamb wave released by the Hunga eruption was so powerful that over a period of six days it circled the Earth multiple times. The team concluded that the wave was on par with that of the infamous Krakatau eruption of 1883 in size and distance traveled, and more than 10 times larger than the one produced by the 1980 Mount St. Helens eruption.

“It shows just how extreme the January 2022 eruption of Hunga volcano actually was,” Cerveny said. “If the Hunga eruption had been primarily above sea-level (like the Krakatau eruption), the effects would have been something not witnessed in nearly 150 years.” 

Krakatau’s upheaval was recorded by around 50 weather barometers stationed around the world, whereas the Hunga eruption was documented by thousands of sensors. This has given scientists an “unparalleled” global dataset for an explosion this size, Matoza says. In the future, he and his colleagues hope to deepen their understanding of Hunga’s eruption by drawing on data from home weather stations belonging to non-scientists around the globe. 

“There’s real potential here to gather up all of this extra data to do even better characterization of this wavefield,” he says.

The vast majority of tsunamis are generated by earthquakes. Early-warning systems are based on these types of waves and can predict their arrival time to within minutes, said Lucie Rolland, a geophysicist at the Côte d’Azur Observatory in Valbonne, France, and another member of Matoza’s team. However, the tsunamis created by the Hunga eruption were significantly different, with the first waves arriving more than two hours earlier than usual. The tsunamis included 4-foot waves that reached the US West Coast.

[Related: From the archives: A 1930s adventure inside an active volcano]

For the second paper, researchers in Japan used mathematical simulations to probe how Lamb waves produced by the eruption could have influenced the ensuing tsunamis. The team drew upon data from barometers, seafloor pressure sensors (which detect tsunamis passing overhead), and global coastal tide gauges in their analysis.

The leading waves were recorded moving at speeds of about 300 meters per second (671 miles per hour). They were followed by waves traveling about 200 to 220 meters per second (447 to 492 miles per hour), which matched typical speeds expected of tsunamis associated with earthquakes, said Tatsuya Kubota, a seismologist at the National Research Institute for Earth Science and Disaster Resilience in Tsukuba, Japan. These disturbances lasted more than three days, which is much longer than earthquake-related tsunamis.

Kubota and his collaborators found that the Lamb wave drove the leading surge of water, while the topographic features of the Pacific seafloor scattered these waves to produce subsequent long-lasting tsunamis. The characteristics of volcanic tsunamis make them more complex and challenging to forecast than earthquake-induced tsunamis, the team concluded.

“One important implication is, I think, [that] we need to incorporate the knowledge of volcanology and meteorology into tsunami science,” Kubota said in an email. 

Rolland noted that her team’s work also suggests that the fast-moving Lamb wave pulse contributed to the unusual tsunamis.

“In absence of correct knowledge on the source, the operational tsunami early-warning systems provided inaccurate estimates of the tsunami threat,” she said in an email. “It is thus paramount to understand the underlying physical mechanisms and to fully explain those peculiar observations and get early-warning systems procedures adapted to the case of explosive volcano eruptions.”





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