Page 20 - Spring 2018

Volcanic Eruption Infrasound
failure and damage to commercial airlines; consequently, volcanic ash clouds lead to extensive flight cancellations, de- lays, and economic losses (e.g., the 2010 eruption of Eyjafjal- lajökull, Iceland).
For example, Alaska is home to 130 potentially active vol- canoes, of which more than 50 have erupted in historical times (Figure 2). Volcanoes in this region are capable of sudden, explosive, ash-cloud forming eruptions, which are potentially hazardous to aircraft along this heavily traveled air corridor. Monitoring of these volcanoes is performed at the Alaska Volcano Observatory (AVO) by integrating mul- tiple ground-based and satellite-monitoring technologies. However, Aleutian Island volcanoes along the western part of Alaska (Figure 2a) in particular represent a formidable monitoring challenge because of their remote locations and harsh weather. Thus, many Aleutian volcanoes are not in- strumented.
Infrasound is atmospheric sound with frequencies between ~0.01 and 20 Hz, that is, below the lower frequency limit of human hearing. The infrasonic frequency band contains the majority of acoustic energy emitted by volcanoes. Over the past two decades, infrasound technology has emerged as a new ground-based method to detect and quantify volcanic erup- tions at both local and remote distances from volcanoes.
The recent progress in volcano-infrasound research has been driven in part by the opening for signature of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in 1996. The treaty calls for a verification regime to be established, of which the International Monitoring System (IMS) is one element. The IMS infrasound network (Figure 1, open in- verted triangles) is designed to detect atmospheric nuclear explosions anywhere on the planet (Christie and Campus, 2010). Construction of the IMS has led to rapid advances in infrasound technology (e.g., improvements in instrumenta- tion, signal-processing methods, and infrasound propaga- tion modeling), which have been transferred and adapted to volcanology.
The IMS infrasound network regularly records signals from large explosive volcanic eruptions worldwide. In addition, numerous other infrasound stations have been deployed near volcanoes or in volcanic regions to study and monitor volcanoes. Sensor acquisition geometries have included net- works of individual infrasound sensors, infrasound arrays, networks of infrasound arrays like the IMS, and colocated seismic and acoustic (seismo-acoustic) stations. For exam- ple, the EarthScope USArray Transportable Array has now
Figure 2. a: Map of historically active Alaska volcanoes and cur- rent infrasound stations. Inset: 210 EarthScope Transportable Array (TA) colocated seismic and infrasound stations spread across Alaska. b: Satellite image of Bogoslof volcano, Alaska, in June 2017. Image courtesy of Alaska Volcano Observatory (AVO)/US Geological Ser- vice (USGS). Image data acquired with the Digital Globe NextView License. The 0.5- to 5-Hz filtered waveforms (c) and spectrogram (in dB re 20 μPa; d) for the January 31, 2017, eruption of Bogoslof as recorded on an infrasound array 60 km away (named OKIF).
been established in Alaska with 210 seismo-acoustic stations at roughly 85 km spacing (Figure 2a, inset), bringing the densest ever seismo-acoustic network to one of the world’s most active volcanic regions.
Explosive Volcanic Eruptions
Volcanism occurs in a great variety of styles, ranging from so-called effusive processes (a passive outpouring of lava onto the Earth’s surface, such as the lava flows that are ex- emplified by Hawaiian volcanoes) to highly explosive (vio- lent, energetic eruptions such as at Mount St. Helens in 1980). Volcanoes erupt explosively because trapped volatiles (mostly water, CO2, and SO2) rapidly come out of solution and undergo huge volume expansions as magma ascends
18 | Acoustics Today | Spring 2018