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

Erupting volcanoes are obviously noisy places, but there are sounds that we do not hear as well. Infrasound is an acoustic wave with a frequency that is too low to be heard by humans.

Acoustic or sound waves audible to humans typically have frequencies ranging between 20 Hertz (Hz), or cycles per second, and 10,000 Hz. Acoustic waves above 10,000 Hz are called ultrasound and are used in medical applications. Infrasound lies at the low-frequency end, with frequencies 20 Hz and below. Animals can often hear infrasound and some animals, like whales and elephants, use infrasound to communicate for great distances.

Infrasound monitoring was developed for detecting nuclear explosions. Nuclear explosions produce infrasound that can propagate large distances and, thus, can be detected by a relatively sparse worldwide network of infrasound sensors. Beside nuclear explosions, there are a number of sources of infrasound, including surf, meteors, and rocket launches. Really, any process that moves air in the atmosphere can produce infrasound.

Most importantly for HVO, there are several sources of infrasound from volcanoes, such as from explosions, lava-lake spattering, rock fall, avalanches and shallow earthquakes. On Kilauea, there are currently two persistent sources of infrasound, one from the lava lake at Halema`uma`u and one from the vent at Pu`u `O`o. The March 2011 Kamoamoa fissure eruption was also a very large source of infrasound.

Infrasound is recorded using instruments that measure atmospheric pressure changes— essentially, low-frequency microphones. Wind is an extremely efficient generator of infrasound energy and is the largest source of noise in infrasound monitoring. To reduce the effect of noise, many sensors are put out in the same approximate location, which is called an array. In infrasound arrays used for nuclear detection, tens or even hundreds of sensors may make up an array. Such an array is extremely expensive and, thus, arrays installed on volcanoes typically have between 4 and 10 sensors.

Infrasound has been measured and studied at Kilauea for several years. The Infrasound Laboratory at the University of Hawai`i (ISLA) led the initial effort. Using ISLA’s experience, and encouraged by the foundation of results from their studies, HVO installed the first of three planned infrasound arrays in late May on the southeast flank of Mauna Loa. Within its first week, the array was able to detect a rock fall within the Halema`uma`u Overlook vent, despite its location 15 km (10 mi) away. The two additional arrays will be installed this summer to the south of Kilauea’s summit and to the north of Pu`u `O`o.

When done, the combined infrasound network will be able to detect and locate fissures and breakouts anywhere on Mauna Loa or Kilauea to within approximately 30 m (100 ft), within about 15 minutes after the start of the eruption. This is an extremely important capability, particularly when the summits and rift zones of Mauna Loa or Kilauea are shrouded in clouds and remote cameras are not effective.

Armed with such information, scientists at HVO will be able to respond faster to eruptions and improve risk assessment. In addition to an enhanced monitoring capability, infrasound observations from HVO’s new instruments and ISLA’s existing instruments will facilitate research on the lava lakes and near-surface volcanic processes at Halema`uma`u and at Pu`u `O`o. We hope this infrasound research will be heard loud and clear in the scientific community.

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Time-lapse movie of Pu‘u ‘O‘o Crater.