Electromagnetic monitoring of the Cumbre Vieja eruption (La Palma, Canary Islands)

Geophysical monitoring is essential to understand the activity and behaviour of volcanoes, especially when new or unusual signs are detected. Monitoring would help to address critical issues, such as the evolution of the magma at depth, which is of particular interest when the volcanic event takes place in populated areas.  Among the geophysical methods, the electromagnetic ones have not been widely used in volcanic monitoring. However, these methods are very sensitive to the presence of fluids and therefore, to the presence of magma, since this will have a significant impact on the electrical resistivity of the subsoil.

Thus, after the onset of the volcanic eruption that began on September, 19, 2021 in the Cumbre Vieja area, on the island of La Palma (Canary Islands), several magnetotelluric stations were installed to perform a volcanic monitoring experiment. The different geophysical stations were installed in the surroundings of the volcanic edifice, as well as in those areas where the epicenters of the major seismic swarms were located. Magnetotelluric stations have been installed for continuous monitoring, recording electric and magnetic fields in the N-S and E-W directions. According to the quality of the data, we have obtained transfer functions for the period range of 0.001 - 100 s.

The aim of this experiment is to analyze the possible variations of the apparent resistivity and phase curves in time. In addition, the electrical resistivity model of the island published in 2020 will be compared with the seismicity that has been occurring since the beginning of the volcanic crisis.

The first preliminary results show slight changes in time and, in comparison with the previous data, this could be related to magmatic movements at depth. Furthermore, the comparison of the seismicity with the previous electrical resistivity model shows that most of the epicenters are located outside the clay cap. This epicenter-resistivity structure relationship is highlighting the considerable difference in the behavior of the clay layer and the surrounding rocks, which is somewhat validating the resistivity model and its interpretation.