Analysis of volcanic activity of Yasur volcano with long range infrasound observation

The atmospheric injection of gas and material produced by an explosive volcanic eruption determines a rapid compression of the atmosphere, which subsequently propagates as longitudinal elastic waves (sound). The size of the source, generally greater than tens of meters, and its duration, longer than few seconds, result into an emitted signal that is particularly rich in low frequency (f < 20 Hz), thus determining an efficient infrasound radiation. Thanks to the low spectral content and the reduced attenuation in the atmosphere, infrasound is capable of propagating for very large distances.

In this study we show how the infrasonic monitoring of volcanoes at regional distances (> 100 km) is efficient in recording and characterizing volcanic events. For the purpose of our study, detections from the Yasur volcano (Tanna Island, Vanuatu) registered at a source-to-receiver distance of 400 km by the IS22 infrasound array, located in New Caledonia and part of the Comprehensive nuclear Test Ban Treaty (CTBT) International Monitoring System (IMS), were studied for a period of eleven years (2008-2018). The predominantly explosive Strombolian activity of this volcano makes it a perfect subject to be studied by infrasound technology.

Detections of infrasound signals from Yasur volcano, that are modulated according to the seasonal variation of stratospheric winds, are corrected for attenuation accounting for real atmospheric specification between the source and the receiver to retrieve the pressure at the source: next, they are used to evaluate long term (yearly) and short term (hourly) variations of activity over the period of analysis. Results are eventually compared with thermal anomalies recorded by the MODIS (MODerate resolution Imaging Spectroradiometer) installed on NASA's Terra and Aqua satellites and computed by the MIROVA hotspot detection system.

We show that even at regional (400 km) distances it is possible to follow the fluctuations of ordinary explosive activity during periods of optimal propagation of infrasonic waves in the atmosphere, In addition, we show that, when the signal is recorded, the time resolution retrieved from the analysis allows following variations of activity at hourly time scale, thus representing a valuable source of information, in particular in areas where local geophysical observation is missing.