Long range infrasound monitoring of Yasur volcano

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 a few seconds, result into an emitted signal that is particularly rich in low frequencies (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 at regional distances (> 100 km) is efficient in recording and characterizing volcanic events. For the purpose of our study, infrasound signal radiated from Yasur volcano (Tanna Island, Vanuatu) was studied for a period of twelve years (January 2008 - December 2019). Signals from Yasur were registered at a source-to-receiver distance of 400 km by the IS22 infrasound array, located in New Caledonia, part of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS). The predominantly explosive Strombolian activity of this volcano makes it a perfect subject to be studied by infrasound technology. Detections of volcanic infrasound are modulated according to the seasonal variation of stratospheric winds and 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 compared with thermal anomalies recorded by the MODerate resolution Imaging Spectroradiometer (MODIS) installed on NASA's Terra and Aqua satellites.We show that even at regional distances (400 km) it is possible to follow the long term (yearly) fluctuations of ordinary explosive activity during periods of optimal propagation of infrasonic waves in the atmosphere. In addition, we show that the time resolution retrieved from the signal analysis allows to follow variations of volcanic activity at hourly time scale, thus representing a valuable source of information, in particular in areas where local geophysical observations are missing.