A mysterious signal came before the biggest Tonga eruption. It was missed

This seismic signal travelled a great distance, indicating an unusually large event was about to happen.

In Short

• The team analysed data from two distant seismic stations
• The seismic wave travelled through the Earth's surface
• It was detected 750 kilometers away from the volcano

A mysterious signal came minutes before the underwater Hunga Tonga-Hunga Ha'apai volcano erupted in the Pacific Ocean. But, it was missed.

A new study published in Geophysical Research Letters suggests that the seismic wave detected 15 minutes before the massive January 2022 eruption could pave the way for early warning systems for remote oceanic volcanoes.

Researchers from the University of Tokyo, led by volcanologist Mie Ichihara and graduate student Takuro Horiuchi, analysed data from two distant seismic stations that recorded a Rayleigh wave - a type of seismic wave that travels through the Earth's surface.

This wave, detected 750 kilometers away from the volcano, is believed to have signaled a precursor event to the eruption.

The team proposes that the seismic wave was caused by a fracture in a weak area of oceanic crust beneath the volcano's caldera wall. This fracture likely allowed seawater and magma to mix explosively above the volcano's subsurface magma chamber, triggering the massive eruption.

"Early warnings are very important for disaster mitigation," Ichihara emphasised, highlighting the potential of such signals to provide crucial time for preparation against tsunamis and other hazards associated with island volcanoes.

The Hunga Tonga-Hunga Ha'apai eruption on January 15, 2022, was a record-breaking event, injecting enormous amounts of water vapour into the stratosphere and generating an unprecedented lightning storm and tsunami.

While the exact start time of the eruption is still debated, the newly discovered Rayleigh wave was detected around 3:45 UTC, approximately 15 minutes before the commonly agreed eruption start time.

Horiuchi noted that while many eruptions are preceded by seismic activity, such signals are typically subtle and only detectable within a few kilometers of the volcano.

In contrast, this seismic signal traveled a great distance, indicating an unusually large event.

The researchers believe this precursor event may have initiated an underground process that ultimately led to the massive eruption. However, they caution that due to the rarity of observed caldera-forming eruptions, especially in oceanic settings, this scenario may not be the only possible explanation.

Despite the challenges in pinpointing the exact causes of such colossal eruptions, this research opens up new possibilities for early warning systems.

The ability to detect these early signals could provide valuable preparation time for island nations and coastal areas facing imminent tsunamis, even when the signals are not felt on the surface.

As Ichihara concluded, "At the time of the eruption, we didn't think of using this kind of analysis in real-time. But maybe the next time that there is a significant eruption underwater, local observatories can recognize it from their data"8.

This new breakthrough could significantly enhance our ability to mitigate the impacts of major volcanic events in the future.