Mayon volcano
Lava runs down the cone of the Mayon volcano, as seen from Legazpi in Albay province- Representational image Charism Sayat/AFP

Listening for infrasound -- inaudible low frequencies that are produced from deep within volcanoes -- can help improve the forecasting of deadly eruptions, scientists have found.

Researchers working in monitoring stations located along the slopes of one of the most active volcanoes in the world — the Villarrica in southern Chile, have recorded infrasound emanating from deep within the mountain. This distinct sound, they say, comes from a roiling lava lake that is in a crater inside the volcano and depending on the activity, it changes.

A report by notes that the study showed exactly how the sound signals a sudden spike in levels inside the lake as well as rapid up and down motions like waves along the rim of the lake. These sound signals were recorded and studied just before a major eruption took place in 2015.

Tracking this sound and other metrics such as seismic readings could help alert residents and tourists who might be in the surrounding regions well before a volcano blows its top, note researchers.

"Our results point to how infrasound could aid in forecasting volcanic eruptions," said study co-author Leighton Watson, a graduate student in the lab of Eric Dunham. He is also an associate professor in the Department of Geophysics of the Stanford School of Earth, Energy & Environmental Sciences. "Infrasound is potentially a key piece of information available to volcanologists to gauge the likelihood of an eruption hours or days ahead."

Villarrica has an altitude of about 9,300 feet and looms over the city of Pucón, a major tourist destination in the summer. The report mentions that the lava lake can sometimes be seen with a faint orange glow in the night sky. The last deadly eruption took place on 3 March 2015 with lava erupting to nearly a mile into the sky.

Infrasound detectors that were installed in the monitoring stations were studied for variations in the sounds before, during and after the eruption, notes the report. During the build-up to the eruption, the data showed that while the duration of the signals decreased, the pitch of the recorded sound increased. Aircrafts flying over the peak recorded the changes in the lava lake inside the volcano and this made it possible for researchers to correlate the height of the lake and the sounds it made.

While the data does show how sounds change before an impending eruption, it is not a universal constant, say the researchers. "Volcanoes are complicated and there is currently no universally applicable means of predicting eruptions. In all likelihood, there never will be," Dunham said. "Instead, we can look to the many indicators of increased volcanic activity, like seismicity, gas emissions, ground deformation, and –as we further demonstrated in this study– infrasound, in order to make robust forecasts of eruptions."

The study was first published in the journal- Geophysical Research Letters.