Temporal changes of seismic wavefield during the 2007 eruptions of Piton de la Fournaise, La Réunion island

Probing the temporal changes in seismic structure of active volcanoes is one of the fundamental way to gain insights into the dynamics of the volcanoes because seismic structure is sensitive to changes in material properties cuased by stress changes, dikings, and others. Seismic random signals such as codas and ambient noise are capable of detecting subtle changes of the seimic properties, as evidenced by growing numbers of applications which successfully demonstrate subtle velocity changes in the subsurface of the Earth. Here we explore the temporal changes of seismic wavefield during the 2007 eruptions of Piton de la Fournaise (PdF). In specific, our goal is to delineate the temporal changes of seismic properties of an area where something other than velocity changes is likely to have happened due to a lateral dike intrusion. We first obtained temporal changes of Green functions of two seismic sites traversing a possible diking region by cross-correlating the two seismograms. The obtained Green functions have drastically changed after the first eruption of PdF on 30 March 2007, when a lateral dike intrusion is likely to have occurred across the baseline of the two seismic sites we processed, leading to distal eruptions between 2-30 April 2007. Indeed, correlation coefficients between the Green functions before the intrusion and those after the intrusion are significantly degraded. Also the Green functions after the intrusion have significatly lower power in frequencies higher than about 2.5 Hz than those before. The power in higher frequencies recovered after distal eruptions ended on 30 April. The observed features cannot be explained only by velocity changes but be explained by attenuation changes to some extent. The recovery of the high frequency content after the eruptions is interpreted to be the solidification of the intruded dike. Our work demonstrate a possibility to extract other information than velocity changes through continuous monitoring of seismic noise.