Integrated geophysical analysis of explosive activity at Yasur volcano, Vanuatu

We present seismic, infrasonic and thermal data collected during a temporary experiment (July 26-Aug 3, 2008) carried on at Yasur volcano (Tanna Island, Vanuatu). At the time of the experiment, activity consisted into intermittent degassing and discrete explosions from three active vents located into two craters. Explosive activity spanned from mild strombolian explosions, dominated by ballistics reaching heights of few hundred meters, to long-lasting (up to 20 seconds) sustained ash emissions, feeding plumes reaching heights of several hundred meters. We deployed a 4-element infrasonic array, co-located with a Guralp CMG-40T broadband seismometer, at a distance of ~700 m from the active craters. A second station, equipped with an infrasonic sensor and an infrared thermometer, was installed on the crater rim and targeted one of the three active vents. Data were collected with 24 bits A/D converters at the sampling frequency of 100 Hz. In addition to the continuous integrated geophysical observation we acquired, between July 30 and August 2, 9h 20min of activity at the craters with a FLIR-A20 Thermocamera at 50 Hz. Timing of all the data was obtained with GPS receivers. Explosive activity at Yasur volcano is recorded seismically as discrete emergent transients, with duration of ~10 seconds and spectral content up to ~10 Hz. Infrasonic observation points to discrete infrasonic transients with amplitude of 20-150 Pa (at 700 m) and duration of 2-10 seconds produced by explosive events and sustained intermittent degassing, with pulses of 1-5 Pa (at 700 m) repeating at a rate of ~2 seconds. Based on the coherence of infrasound across the array we locate explosive and degassing activity, leading to a robust description of the infrasound emitted by the three active vents. Thermal imagery of 423 explosions, recorded during the observation period of 9h 20min, leads to a detailed description of the plume emission and ascent for explosions from the 3 active vents, providing information on plume initial velocities, emission rates and plume evolution. Analysis of thermal imagery indicates a mean rate of 44.5 explosions per hour and emission velocities of 50 to 250 m/s.