4 April 2018
By Nanci Bompey
High-resolution recordings of the powerful infrasound waves generated by an eruption at Ecuador’s Tungurahua volcano have given scientists a rare view inside the activity at the volcano’s mouth.
The acoustic waves generated by the July 2013 eruption were one of the most powerful volcanic infrasound recordings ever captured. The low-frequency infrasound waves from the eruption are too low for human ears to hear but were as powerful as waves one meter (three feet) away from a jet engine.
Recording such powerful infrasound waves allowed scientists to better understand the activity at the volcano’s mouth, or vent, a place difficult to observe because it is obscured by the volcano’s ash cloud. From the infrasound waves, scientists were able to calculate the amount of gas generated by the eruption, which controls the volcano’s plume, and how the volcano’s activity changed as the eruption progressed.
Understanding what is happening at the volcano’s vent and plume at the time of the eruption could improve warnings for people living nearby, according to Jake Anderson, a graduate student at Boise State University in Idaho and lead author of a new study detailing the new research in Geophysical Research Letters, a journal of the American Geophysical Union. A related study in Geophysical Research Letters co-authored by Anderson shows infrasound could also be used to monitor changes in a lava lake, which can precede an eruption.
“It is currently pretty tough to watch an eruption visually because with the ash cloud you can’t see very well, so we need to turn to geophysics to work out what is happening there,” Anderson said. There are other geophysical tools, like seismometers, that can be used to monitor an eruption, “but if you want to unambiguously detect what is happening at the vent, infrasound is the best way to do it,” he added.
A powerful eruption
Tungurahua volcano woke up in 1999 and continued to erupt a few times a year for the next 17 years, causing evacuations and worry in nearby communities. Because of its activity, Tungurahua has been an area of intense monitoring and research.
For their part, Anderson and his colleagues set out to record the infrasound waves generated by the powerful eruptions. The infrasound waves generated by the eruption, which were about 100 times below the frequency of human hearing, can give scientists insight into the volcano’s vent.
In 2013, the team set up a temporary network of sensors to record the infrasound closer to the volcano’s vent than typical, more permanent sensors. Although there is a greater risk of losing the instruments, setting up sensors so close to the vent allows scientists to record the waves without distortion that happens when the waves travel to sensors further away.
On July 14, 2013, Tungurahua erupted. The pressure from the explosion was heard 180 kilometers (112 miles) away and its plume reached 8.3 kilometers (5.2 miles) above the crater. The initial eruption was followed by occasional small eruptions over the next 23 days.
The eruption was one of the most powerful explosions of the volcano since continuous, comprehensive monitoring began in 2006, and one of the most powerful volcanic eruptions recorded with a nearby sensor network anywhere in the world, according to the new study.
“If we could hear (the infrasound waves), it would be absolutely deafening; the loudest thing you would ever hear in your life,” Anderson said.
Seeing inside the plume
The sensors Anderson and his colleagues set up recorded the infrasound waves generated by the eruption and enabled them to reconstruct the eruption at the vent.
They found pressured gas pulses opened the volcano’s vent over a 2-second period, resulting in the rapid expulsion of at least half a cubic kilometer (0.1 cubic miles) of gas and ash – a massive amount and one of the best estimates from such a large eruption, according to Anderson.
Volcanic lightning – which was shown as glitches in their acoustic data – began 25 seconds after the explosion. Ash and gas continued to be spewed from the volcano, producing continuous infrasound waves starting 50 seconds after the explosion and lasting for about 19 minutes. The volcanic lightning stopped after about 20 minutes. At the same time, activity at the volcano’s vent transitioned to pulses of gas being emitted from the vent, producing pulses of infrasound lasting for 30 minutes.
Recording close to the eruption allowed the researchers to see these different infrasound waves and better understand the finer details of what was happening at the vent. In the 2013 eruption, for example, the infrasound recordings allowed scientists to see the point where activity at the volcano changed from one type of venting to another. Vent activity has implications for hazards, and this information could be useful to monitors and civil defense for alerting threatened communities, Anderson said.
“Near-vent infrasound monitoring can tell us things about what the volcano is doing that we can’t get from any other data stream,” he said.
— Nanci Bompey is the manager of the Public Information department at AGU. Follow her on twitter at @nbompey.