S53D-02
A Precursory Phase to a Sudden Enhanced Activity at Yasur volcano (Vanuatu) : Insights from Simultaneous Infrasonic and Seismic Records

Friday, 18 December 2015: 13:55
307 (Moscone South)
Sylvie Vergniolle1, Christelle Zielinski1, Jean Battaglia2, Jean-Philippe Metaxian3, Philipson Bani3, Alexis LE Pichon4, Michel Lardy5, Philippe Millier6, Michel Frogneux5, Francis Gallois5, Pascal Herry6, Sylvain Todman7 and Esline Garaebiti7, (1)Institut de Physique du Globe de Paris, Paris, France, (2)Laboratoire Magmas et Volcans, Clermont Ferrand, France, (3)IRD, Laboratoire Magma et volcans, Clermont-Ferrand, France, (4)CEA Commissariat à l'Energie Atomique DAM, Arpajon Cedex, France, (5)Institut de Recherche pour le Developpement, Noumea, New Caledonia, (6)Commissariat a l’Energie Atomique, Bruyeres le chatel, France, (7)Vanuatu Meteorology and Geohazards Department, Port Vila, Vanuatu
Abstract:
The permanent activity at Yasur (Vanuatu), characterised by a close series of Strombolian explosions, is analysed using simultaneous infrasonic and seismic recordings (6-25 Nov 2008) close to the vents. The RMS amplitudes per hour, the number of explosions and the peak-to-peak amplitudes of each signal show that the initial quiet phase (11 days) is followed by a precursory phase (7 days) prior to an enhanced activity (17 hours). Three periods exist during the strong activity: (1) a rapid increase leading to the paroxysm (3 hours), (2) a first (5 hours) and (3) a second decrease (9 hours), each having an excellent correlation between seismic and infrasonic RMS amplitudes per hour (correlation coefficient > 0.96) when using the band associated to explosions (1-5 Hz and 1.8-4 Hz for seismic and infrsonic recordings, respectively).

The ratio between infrasonic and seismic RMS amplitudes, assumed to be a proxy for the magma level, increases strongly during the week before the paroxysm. This is explained by the arrival of an additional gas flux at the top of the reservoir. The foam accumulated there, whose partial coalescence and spreading towards the conduit are responsible for the permanent Strombolian activity, thickens. This enhances both the viscous massive foam coalescence and the foam spreading. This leads to an increase in the gas flux in the conduit, ultimately responsible for the formation of a shallow foam at the surface. This foam acts as a viscous cap overlying the magma column, thereby increasing the radiated infrasonic pressure and the strength of the explosions. The first decrease in the relationship between infrasonic and seismic RMS amplitudes is associated with the stopping of the additionnal gas flux in the magma reservoir and the rapid decrease of the top of the magma column due to the previous intense degassing. The second decrease corresponds to the time neccessary to restore the convective motions in the conduit at their normal velocities.

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