The study proposes that this seismic wave resulted from a sudden fracture within a weak portion of the oceanic crust beneath the volcano's caldera wall. This rupture, researchers explain, allowed seawater to mix violently with magma in a space near the volcano's magma chamber, creating conditions that ultimately triggered the eruption.
These findings were published in 'Geophysical Research Letters', an open-access journal by the American Geophysical Union (AGU), which focuses on impactful, short-format studies across Earth and space sciences.
Building on earlier research on remote volcano monitoring, the study identified a specific type of seismic wave, known as a Rayleigh wave, which was detected approximately 750 kilometers (466 miles) from the eruption site. This discovery highlights the potential of seismic waves as early indicators of eruptions.
"Early warnings are very important for disaster mitigation," noted Mie Ichihara, a volcanologist at the University of Tokyo and coauthor of the study. "Island volcanoes can generate tsunamis, which are a significant hazard."
Unseen Warning Before a Major Event
Hunga Tonga-Hunga Ha'apai, a submarine volcano in Tonga, owes its existence to the tectonic subduction of the Pacific Plate under the Australian Plate, a geological process that fuels volcanic eruptions.
On January 15, 2022, the volcano erupted with astonishing force, sending vast quantities of water vapor high into the atmosphere, sparking a massive lightning storm, and triggering a tsunami. This dramatic event followed a smaller eruption on January 14 and a month of increased volcanic activity.
The exact eruption start time remains debated, though most agree it was shortly after 4:00 UTC. The study records a Rayleigh wave beginning around 3:45 UTC, implying that it may have marked the eruption's initial phase.
The researchers analyzed seismic data from stations in Fiji and Futuna, which registered the Rayleigh wave undetectable to humans. While Rayleigh waves often accompany volcanic eruptions and earthquakes, this particular signal likely indicated an early precursor to the eruption.
"Many eruptions are preceded by seismic activity," explained Takuro Horiuchi, lead author and volcanology graduate student at the University of Tokyo. "However, such seismic signals are subtle and only detected within several kilometers of the volcano."
This unique signal, which traveled a significant distance, points to a large-scale seismic event. "We believe unusually large movements started at the time of the precursor," Horiuchi added.
Understanding Caldera-Forming Events
While the precise trigger of this caldera-forming eruption may remain elusive, Ichihara believes it was not an instantaneous occurrence. Instead, she suggests that this precursor wave marked the beginning of a subsurface process culminating in the eruption.
"There are very few observed caldera-forming eruptions, and there are even fewer witnessed caldera-forming eruptions in the ocean," Ichihara said. "This gives one scenario about the processes leading to caldera formation, but I wouldn't say that this is the only scenario."
Still, recognizing such early eruption signals could offer precious preparation time for island nations and coastal communities, particularly when a seismic warning goes unnoticed on the surface.
"At the time of the eruption, we didn't think of using this kind of analysis in real-time," Ichihara explained. "But maybe the next time that there is a significant eruption underwater, local observatories can recognize it from their data."
Research Report:A seismic precursor 15 minutes before the giant eruption of Hunga Tonga-Hunga Ha'apai volcano on January 15, 2022
Related Links
Earthquake Research Institute, The University of Tokyo
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