(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

If you visit Hawaiʻi Volcanoes National Park’s Jaggar Museum Overlook when the wind is calm, you might be able to hear the sounds of gas bubbles bursting and lava splashing in the Halemaʻumaʻu lava lake at the summit of Kīlauea. What you hear is only part of a rich chorus of sounds emitted from many processes near the surface of an active lava lake.

While some lava lake sounds are audible, most of them are at frequencies below what humans can hear, called infrasound. As with the electromagnetic spectrum, in which long-wavelength infrared is just below the visible light range, infrasound is at frequencies below 20 Hz (Hertz, a measure of audio frequency), which human ears cannot hear.

We know that many other natural processes make sounds that travel through both the ground and the air. For example, atmospheric sounds, such as thunder, can transfer into the ground and produce seismic waves. Conversely, small shallow earthquakes can produce low-frequency booming sounds when seismic waves reach the surface and vibrate the air. In fact, P-waves, the fastest type of seismic waves, are just sound waves traveling through the solid Earth.

Active volcanoes produce abundant sounds at or near Earth’s surface. So, it can be beneficial to record those sounds using both seismometers for the seismic waves and microbarometers for the infrasound. Microbarometers are similar to sensors that measure pressure changes from passing weather fronts, but detect much smaller-scale changes in pressure.

So, what’s the connection to lava lakes? At Halemʻaumʻau, the loudest sounds are about 1 Hz and can only be captured with dedicated infrasound recording equipment. A frequency of 1 Hz is about the same as that of strong seismic tremor produced at the volcano’s lava lake and within the magma plumbing system. Seismic and infrasound sensors record different versions of this tremor and can be used together to better understand it.

One important difference between seismic and infrasound recordings is the pathway between the source and the recorder. Imagine all the layers of old lava flows off which a seismic wave echoes as it travels through the ground. Each of those echoes arrives at the seismometer at a different time and may result in a complex signal even if the source is simple.

On the other hand, the sound wave in the air has a much simpler path, as long as it doesn’t have too far to go. For a source that sends waves through both the ground and the air, this means that infrasound signals are often much easier to interpret.

At Kīlauea’s summit lava lake, there are times when each big bubble burst can be distinguished individually on an infrasound recording, but the overlapping seismic recordings of the same processes are much too complex to interpret alone. In this way, joint recordings of waves through the air and the ground can be used in the identification of small events in the seismograms.

Another way infrasound and seismic data can be used together is in monitoring the rise and fall of the lava lake. Since sound waves travel more slowly through the air than through the earth, the change in source location as the lake goes up or down means that the time it takes an infrasound signal to arrive at the recorders will change more than the change in time for the seismic wave. Using a little algebra, the change in the depth of the lava lake can be easily calculated. This is especially useful at volcanoes where the lava surface is not visible and cannot be measured more directly.

Infrasound has many applications on volcanoes beyond studies of lava lakes, as described in a previous Volcano Watch (volcanoes.usgs.gov/observatori… ). In particular, infrasound can aid monitoring by continually tracking the directions from which sounds originate, potentially alerting scientists to the onset of new eruptive activity.

It takes a wide array of sensors to monitor an active lava lake. The ability to capture sounds we can’t hear provides a wealth of information we wouldn’t know we were missing.

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