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.

Description: In recent years the use of Distributed Acoustic Sensing (DAS) in seismology is gaining extensive usage in different applications. A High-Fidelity DAS system (HDAS) was deployed during the 2021 Tajogaite eruption on Cumbre Volcano (La Palma, Canary Islands), allowing the recording of most of the syn-eruptive and post-eruptive seismicity. The eruption lasted from Sep. 19th until Dec. 13th of 2021. The HDAS was installed on 19th Oct. 2021 and is still operating. The HDAS was installed around 10 km from the eruptive vent and was connected to a submarine fibre optic cable directed toward Tenerife Island. Since then, the HDAS has been recording seismic with a temporal sampling rate of 100 Hz and a spatial sampling rate of 10m for a total length of 30 (first phase) and 50 km using Raman Amplification (last period). The HDAS recorded thousands of local earthquakes as well as regional and teleseismic events. It was revealed to be an excellent tool for volcanic monitoring, allowing a better location of deeper events which location was made difficult by the small aperture of the seismic network of La Palma. The HDAS was also able to record the low-frequency (〈1 Hz) component of the volcanic tremor up to a distance of tens of kilometres from the volcano. We show how, using array-like techniques, it is possible to identify and separate the volcanic tremor signals from the oceanic ambient noise. In this work, we demonstrate the effectiveness of using DAS as a real-time volcano monitoring tool.

Description: The joint analysis of multiparametric datasets in geophysics and, in general, in geosciences is often challenging due to the highly different measurement types. During the last decades, data mining techniques have been subject to intense development, which allows the detection and characterizing of “hidden patterns” within complex datasets. One of the most successful and widely used techniques is the Independent Component Analysis (ICA) which allows for identifying spatio-temporal patterns related to independent sources from dense geospatial datasets. In this work, we apply an extension of the ICA named Independent Vector Analysis (IVA) to analyze a multiparametric dataset of spontaneous potential, CO2 and H2S flux and thermal gradient measurement realized in the crater of Mt. Teide (Tenerife, Canary Islands), from 2020 to 2023. While ICA allows studying spatio-temporal patterns of a single quantity, IVA allows dealing with means multiparametric measurements realized on a single point, which means using vector data instead of a simple scalar. The relationship between spontaneous potential and gas emission is well known and testified by numerous case studies. In this work, however, we exploit for the first time this quantitative approach to separate and characterize endogenous and external factors in this dataset. The approach we propose in this work has a broader application to repeated multiparametric geophysical surveys and in combining geophysical datasets with other kinds of data.

Description: To better understand the shallow structure of El Hierro island, we determined a 3D S-wave velocity model using Ambient Noise Tomography (ANT). We exploited a dataset recorded by 21 broadband seismic stations deployed at El Hierro island in two surveys realized in 2015 and 2021. This dataset allowed us to obtain empirical Green’s functions by cross-correlating seismic ambient noise signals and retrieving 105 dispersion curves using the frequency-time analysis (FTAN). Then we obtained 2-D Rayleigh wave group velocity maps for periods between 0.6 s and 2.6 s through a non-linear multiscale inversion (Cabrera-Pérez et al, 2021). Finally, we performed depth inversion through a Bayesian transdimensional approach to obtain a 3-D S-wave velocity model. The obtained ANT model is merged with a local earthquake tomography model (García-Yeguas et al, 2014). Our study highlights six relevant seismic velocity anomalies. We observed the presence of three high-velocity zones located in the eastern, western and northern parts of the island, which could be related to intrusive bodies possibly associated with the formation of El Hierro island. We also observed three low-velocity anomalies in the northern and southern parts. The anomaly in the North of the island could be related to loose deposits generated by the El Golfo valley megalandslide. The anomalies in the South could be related to porous and highly fractured materials produced during the more recent volcanic episodes.

Description: Volcanic eruptions are key scenarios for developing new tools for volcanic monitoring. Among the geophysical techniques, electromagnetic methods are not extensively used in volcano monitoring. However, these methods are very sensitive to temporal changes in the subsoil due to the movement of fluids, which strongly affects the electrical resistivity of the subsoil. During the volcanic eruption that started on the island of La Palma on September 19th, 2021, and the subsequent post-eruptive process, we deployed several magnetotelluric (MT) stations in the vicinity of the new volcano to test the performances of such instrumentation as an effective volcanic monitoring tool. Magnetotelluric stations have been installed for continuous monitoring, recording electric and magnetic fields along the N-S and E-W directions. We have obtained good quality transfer functions for the period range of 0.01 - 100 s. The main objective of this MT experiment was to detect possible variations of the apparent resistivity and phase curves and to compare them with other geophysical and geochemical studies conducted in the same area. The preliminary results show slight changes in the resistivity over time that could be related to the development of a shallow hydrothermal system around the volcano. Furthermore, in comparing the pre-eruptive 3D resistivity model of the island, we observed that regions with low resistivity were almost aseismic, possibly due to the presence of ductile rocks like clays.

Description: The receiver function analysis (RF) is a well-established method to investigate the crustal and upper mantle structure in different geodynamic settings. It is highly sensitive to seismic discontinuities and provides information about P- and S-wave velocities beneath a seismic station. This work aims to study the crust and the upper mantle of La Palma Island up to 50 km of depth by using RF analysis. La Palma is one of the youngest oceanic volcanic islands of the Canary archipelago, with a complex internal structure that makes applying conventional RF challenging. The island was affected in 2021 by the devastating Tajogaite eruption which occurred on the Cumbre Vieja volcano. For this study, we obtained RFs of five stations using multi-taper deconvolution. After that, we applied the transdimensional approach of Bodin et al. (2012) to determine 1D profiles of P- and S-wave velocities and the probability of a discontinuity beneath each station. Each 1D model was interpolated to build a N-S trending seismic section that allowed us to correlate the different discontinuities. We observe a Low-Velocity Zone in the upper mantle beneath the Cumbre Vieja volcano and four different layers with a significant change in their Vp/Vs ratios. These results are compared with the seismicity recorded beneath the island from 2017 until the pre-eruptive phase of the Tajogaite eruption. Hypocenters are mainly located at the base of the crust (10-15 km deep) and in the upper mantle (20-30 km deep), possibly related to the presence of two subcrustal magmatic reservoirs.