According to new research from the University of Washington, signals from the ionosphere could help improve tsunami forecasting and, in the future, track ash plumes and other impacts after a volcanic eruption.
The new study investigated the Hunga Tonga-Hunga Ha’apai eruption that occurred earlier this year in the South Pacific. The volcanic eruption on January 15, 2022, was the largest recorded by modern equipment.
The area was covered in ash, and on the island of Tonga, a tsunami wave caused damage and killed at least three people. It also had far-reaching consequences.
Most Powerful Eruption Since Krakatau
In more than a century, no volcanic eruption has resulted in a global-scale tsunami. The tsunami wave caused by the underwater eruption was initially predicted to be only a regional hazard. Instead, the wave made it all the way to Peru, where two people drowned.
Many aspects of this volcanic eruption, which was the most powerful since Krakatau’s eruption in 1883, were unanticipated.
“We used a new monitoring technique to understand what happened here and learn how we could monitor future natural hazard,”
said lead author Jessica Ghent, a UW doctoral student in Earth and space sciences.
Ionospheric Tracking Of Pressure Waves
Because tsunamis occur so infrequently, forecast models that rely on a small number of tide gauges and ocean sensors are still being developed. This research is a part of a growing body of work investigating how to follow events on the ground using GPS signals that are transmitted through the atmosphere.
A large earthquake, or in this case, a massive volcanic eruption, causes pressure waves to form in the atmosphere. The particles are disturbed as these pressure waves pass through the ionosphere, a zone of about 50 to 400 miles altitude where electrons and ions float freely.
GPS satellites returning coordinates to Earth send a slightly altered radio signal that tracks the disturbance.
“Other groups have been looking at the ionosphere to monitor tsunamis. We are interested in applying it for volcanology. This Tonga eruption kicked our research into overdrive. There was a big volcanic eruption and a tsunami. Normally you’d study one or the other,”
said co-author Brendan Crowell, a UW research scientist in Earth and space sciences.
Sonic Boom Tsunami Amplification
Mapview of ionospheric disturbance arrivals over the southwestern Pacific for satellites G10 and G23.
The general direction of satellite motion is from southwest to northeast between the time of eruption, 04:14 UTC, and 12:00 UTC on 15 January 2022. Yellow boxes represent the positions of Deep-ocean Assessment and Reporting of Tsunamis buoys for which a first peak arrival is available. The red triangle denotes the location of Hunga Tonga-Hunga Ha’apai (HTHH). Green circles indicate the locations of Global Navigation Satellite System stations that are discussed herein. Total electron content units (TECu) are saturated beyond +/− 0.4 to emphasize the locations of the strongest signals.
Credit: Geophysical Research Letters (2022). DOI: 10.1029/2022GL100145
The researchers used 818 ground stations in the Global Navigation Satellite System, a global network that includes GPS and other satellites, to measure the atmospheric disturbance in the hours following the eruption for the new study.
The results support the hypothesis that the sonic boom generated by the volcanic explosion enlarged and accelerated the tsunami wave. The ocean wave received a boost from the pressure wave generated by the eruption.
Due to the rarity of tsunamis caused by volcanoes, this additional push was not included in the initial tsunami forecasts, researchers said.
“Tsunamis typically can travel in the open ocean at 220 meters per second, or 500 miles per hour. Based on our data, this tsunami wave was moving at 310 meters per second, or 700 miles per hour,”
Ghent said.
Signal Separation
The authors were able to distinguish between distinct aspects of the eruption, including the acoustic sound wave, the ocean wave, and other types of pressure waves, and validate their accuracy using data from ground-based observation stations.
“The separation of these signals, from the acoustic sound wave to the tsunami, was what we had set out to find. From a hazards-monitoring perspective, it validates our hope for what we can use the ionosphere for. This unusual event gives us confidence that we might someday use the ionosphere to monitor hazards in real time,”
Ghent said.
Ghent and Crowell believe the Global Navigation Satellite System signals could be used in other ways to accurately track volcanic ash plumes, despite the fact that the Tonga eruption did not eject a significant amount of ash relative to the size of the event.
Because ground-based monitoring is difficult in the Pacific Northwest and other areas, looking up to monitor volcanoes and tsunamis is appealing. Sensors must be maintained and repaired; snow and ice can interfere with signals or cause damage; and access to monitoring stations may be difficult. Also, because goats like salt, wild mountain goats sometimes eat the cables of ground instruments.
“If you have a way to monitor an area without actually being there, you’re really opening the door to being able to monitor it all year long and help keep people safe around the world,”
Ghent said.
Reference: Ghent, J. N., & Crowell, B. W. (2022). Spectral characteristics of ionospheric disturbances over the southwestern Pacific from the 15 January 2022 Tonga eruption and tsunami. Geophysical Research Letters, 49, e2022GL100145.