Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

Small-aperture array measurements of seismic noise at seven sites around Yasur volcano (Vanuatu) are performed to estimate the VP and VS velocities of the shallow structure. The spatial autocorrelation (SPAC) and the frequency–wavenumber (f–k) methods are used to determine Rayleigh phase velocity dispersion curves. Phase velocities computed with the SPAC method vary between 580 m/s and 960 m/s at 1 Hz and between 270 m/s and 420 m/s at 15 Hz. F–k dispersion curves show velocities of 300–340 m/s and 800–940 m/s at 1 Hz and 200–230 m/s at 15 Hz. An inversion technique based on the use of the neighbourhood algorithm has been used to calculate the one-dimensional velocity model at each site. Velocity models reach 200 m deep and mainly contain two layers and a half-space. For sites close to the Siwi caldera rims, comparisons with geology and hydrothermal system studies suggest that the two layers highlighted in models may correspond to two large pyroclastic sequences related to caldera collapses based on the flank of an old volcano. Results obtained for the other three sites, located inside the caldera, show the influence of the hydrothermal system on P- and S-wave velocities. For these sites, fluid circulation inside the volcanic deposits causes lower velocities at depth. To obtain a near-surface velocity model of the volcanic structure, each 1D velocity model is spatially extrapolated according to the surface geology. Results highlight four distinct areas, the Siwi caldera edges with high velocities and the resurgent block, the ash plain and the Yasur edifice with lower velocities at depth.