What is infrasound?
When a volcano erupts, it releases energy into the ground in the form of seismic waves and into the atmosphere in the form of acoustic (sound) waves (Figure 1). The majority of the sound from volcanoes is low frequency (below 20 Hz, the threshold of human hearing) and is termed infrasound. Recent research has demonstrated how infrasound can be used to detect, locate, characterize, and quantify volcanic eruptions, providing a valuable tool for volcano monitoring.
Infrasound waves travel at the speed of sound, approximately 340 m/s (760 mph) at sea level, thus taking about 15 minutes to travel every 300 km (185 miles). The propagation velocity of infrasound waves is about 10 times slower than seismic waves. This velocity is determined by the temperature and wind structure in the atmosphere, and therefore a detailed knowledge of the atmosphere and the prevailing winds is necessary to understand the long-range propagation of sound. Other sources of infrasound include wind, surf, meteors, glaciers, thunder, and aircraft. Because there are many causes of infrasound, information is needed about the direction, amplitude, duration, and frequency content of infrasound waves to determine what specifically generated the signal.
Infrasound Monitoring by AVO
Monitoring volcanic eruptions in Alaska is challenging due to the remoteness of many of the volcanoes, making a local monitoring network (e.g. seismic) difficult to establish and maintain. Cloudy weather and delays in satellite image acquisition limit how quickly satellites may detect eruptions. Because infrasound is not affected by cloud-cover and can travel long distances, it is a useful tool to detect and monitor volcanic eruptions in Alaska and other locations worldwide. When a volcano produces infrasound, it provides clear evidence that the volcanic vent is open to the atmosphere. Volcanic seismicity is attributed primarily to fracturing rock and fluid movement beneath the surface, so combining infrasound and seismic data helps determine whether a volcano is erupting or whether the activity is confined below the surface.
Nearly all types of volcanic eruptions produce infrasound, and infrasound from large, explosive eruptions can travel up to thousands of miles and be recorded on sensitive infrasonic microphones. AVO, in conjunction with the Geophysical Institute of the University of Alaska Fairbanks (UAF-GI), has a number of infrasound stations deployed in Alaska and the Aleutian Islands (Figure 2). As of December 2015, AVO has installed infrasound sensors at Augustine, Akutan, Cleveland, Okmok, Makushin, and Pavlof volcanoes, as well as one near the town of Adak. The UAF-GI operates infrasound stations in Fairbanks and Dillingham, Alaska, as well as a number of other locations worldwide. Seismometers also occasionally detect infrasound waves, as the sound energy may shake the ground near the seismometer creating a “ground-coupled airwave”. Scientists at AVO and the UAF-GI analyze the infrasound and ground-coupled airwave data to determine if a volcano has erupted, as well as more detailed information on the eruption itself. These analyses help provide timely warnings of eruptions that can help mitigate volcanic hazards, particularly to aviation.
Infrasound sensors (microphones) are designed to detect very small acoustic (pressure) waves in the atmosphere. Multiple types of infrasound sensors exist and are divided into two categories: absolute and differential pressure sensors. Absolute sensors record very small changes in background atmospheric pressure, while differential sensors output small changes in pressure relative to a reference pressure. After the pressure wave is recorded, it is digitized by attached electronics and transmitted to AVO via radio, Internet, or satellite communications (similar to a seismic station). AVO often deploys infrasound microphones in groups, called arrays, and uses the relative arrival times of acoustic waves on each sensor to determine where the sound is coming from (Figure 3). The inset photo in Figure 3 shows an example array deployed near Cleveland volcano.
Types of infrasound signals from volcanoes
Volcanoes erupt in a variety of styles, and each eruption style produces different and often unique infrasound signals. Infrasonic signals commonly detected include explosions, tremor,jetting and passive degassing. Figure 4 shows some example infrasound waveforms from explosions from various volcanoes around the world, including a large explosion from Augustine Volcano in January 2006 (Figure 4f). Most volcanic explosions start with a short duration pressure wave consisting of a sinusoidal increase and decrease in pressure, although the size of the pressure waves can vary widely. This is followed by lower amplitude, sustained infrasound that can last seconds to minutes.
Another principal type of volcano infrasound is termed “tremor”. Infrasonic tremor results from the continuous perturbation of the atmosphere that can last from tens of seconds to years.
Volcanic jet noise, or jetting, is similar to infrasonic tremor. The lower portion of a large volcanic eruption column is hypothesized to produce sound similar to a jet engine.
Examples of recent infrasound signals from Alaskan Volcanoes
Cleveland volcano, 2015
Infrasound and seismic monitoring stations were first deployed on Cleveland volcano in August 2014, and have since been used to detect explosions, earthquakes, passive degassing and rockfalls from the volcano. Prior to 2014, distant infrasound microphones and seismometers in Alaska and the Aleutian Islands were used to detect explosions from the volcano. Figure 5 shows example seismic (top) and infrasound (bottom) signals from the 21 July 2015 eruption of Cleveland volcano.
Kasatochi volcano, 2008
The large eruption of Kasatochi volcano in 2008 produced infrasonic jetting recorded globally. Figure 6 shows the infrasound waveform (top panel) and spectrogram (frequency content as a function of time) from the Kasatochi eruption on August 8, 2008. These signals were recorded at the IS53 infrasound array in Fairbanks, over 1000 miles away. The sound from Kasatochi has very similar characteristics to the sound from a jet engine
Pavlof Volcano, 2013
Pavlof Volcano frequently erupts and produces explosions recorded on seismometers and infrasound sensors. The figure below shows A) waveforms and B) spectrograms for a Pavlof explosion in May 2013. The first four panels are ground-coupled acoustic waves on nearby seismometers, while the bottom panel is acoustic waves recorded on the Dillingham, AK infrasound array 460 km (286 mi) away. The acoustic propagation time from Pavlof to Dillingham has been removed. Figure from Waythomas and others (2014).