ALBERT

Description: Time-lapse monitoring of seismic velocity at a volcanic area can provide important information about the dynamics of the volcanic system and its temporal variability. One standard technique to monitor small changes in the medium is ambient noise interferometry. This technique is based on quantifying relative velocity variations (dv/v) by measuring changes in the ambient noise cross-correlation signal phase. In this study, we demonstrate that together with velocity variations, the study of changes in seismic attenuation (Qc) extracted from ambient noise cross-correlation could be relevant for volcano monitoring. The Qc is determined using the lapse-time dependence method (Calvet and Margerin, 2013), in which, Qc is measured as a function of the coda window length for different onsets of the ambient-noise cross-correlation coda. We apply this technique to the 2021 Cumbre Vieja eruption in La Palma (Canary Islands), which started on Sept. 19th and had a significant social and scientific impact. First, we determine a reference Qc model of the Cumbre Vieja volcano using data from Aug. 1st to 31st, 2021. Then, we analyze the spatio-temporal Qc variations during the nineteen days preceding the eruption, and we compare our results with previously obtained dv/v results. We observe an increase of Qc during the pre-eruptive phase, corresponding to a dv/v decrease. We hypothesize that the observed correlated Qc increase and dv/v drop could be explained by the ascent of hydrothermal fluids towards the surface before the eruption.