Infrasound Detection of Volcanic Explosions
Posted: July 26, 2012
Monitoring volcanic eruptions in Alaska is often challenging due to the remoteness of many of the volcanoes in the region, making a local monitoring network difficult to establish and maintain. Cloudy weather also regularly hinders the ability of satellites to detect eruptions. Recent studies have demonstrated how infrasound, or low frequency sound waves, can be used to detect, locate, characterize, and quantify volcanic eruptions.
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. The majority of the sound from volcanoes is low frequency (below 20 Hz, the lower threshold of human hearing) and is termed infrasound. Nearly all types of volcanic eruptions produce infrasound, and infrasound from large, explosive eruptions can travel up to thousands of miles due to the low amount of energy loss in the atmosphere. This is in contrast to seismic waves in the Earth, which encounter considerably more energy loss. The Alaska Volcano Observatory (AVO), in conjunction with the Geophysical Institute (GI) of the University of Alaska Fairbanks, has a number of infrasound stations deployed in Alaska. Sensitive microphones at these stations record infrasound. 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 GI analyze the infrasound and ground-coupled airwave data to determine if a volcano has erupted.
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). Their propagation is determined by the temperature and wind structure ofin the atmosphere, and therefore a detailed knowledge of the atmosphere and the prevailing winds is necessary to understand the long-range propagation of sound. In addition, strong winds on the ground can create high noise conditions and potentially obscure the infrasonic signal.
Because infrasound is not affected by cloud-cover and can travel long distances, it is a useful tool to monitor volcanic eruptions in Alaska. For example, the recent eruptions of Redoubt Volcano in 2009 and Kasatochi and Okmok Volcanoes in 2008 all produced significant infrasound detected in Alaska and up to 4000 miles away. The ongoing eruption of Cleveland Volcano has also been detected by infrasound microphones and seismometers in Alaska and the Aleutian Islands. In fact, distant infrasound recordings and ground-coupled airwaves are often the only data used to detect explosions from Cleveland Volcano due to its remoteness, lack of a local seismic network, and consistently poor weather.