Tomographic imaging of the magmatic system feeding the 2021 Cumbre Vieja eruption (La Palma, Canary Islands).

The 2021 Cumbre Vieja surprised the worldwide volcanological community for its peculiar, unexpected features. Among these are the quite explosive character, even having the erupted magma a femic composition, the long duration (almost three months) and the huge erupted volume (more than 200 Mm³). The eruption was preceded by seismicity starting in Oct. 2017. However, the genuine precursory seismicity began only eight days before the eruption, with an evident upward migration of hypocenters. During the eruption, the seismicity mainly was concentrated at a subcrustal (10-15 km) and an upper mantle (20-25) depth.

Before and during the eruption, we collected a dataset of 11,349 earthquakes recorded from 7 October 2017 to 13 to December 2021 with 140,078 P wave and 155,231 S wave picks. We performed a high-resolution traveltime tomography, obtaining a three-dimensional P and S-wave velocity model up to a depth of about 25 km. The tomographic models evidence the presence of various interesting structures. At shallow depth (< 3 km), we identified a localized low-velocity anomaly interpreted as a volume of hydrothermal alteration. The Moho shows a complex geometry, with an upwelling beneath Cumbre Vieja volcano up to 10 km depth. Finally,  a large deep volume (> 400 km³) is characterized by high Vp/Vs values. This volume can be possibly related to the main magmatic reservoir feeding the eruption.

The tridimensional velocity model also allowed for a precise relocation of the seismicity, providing an interesting insight into the evolution of the eruption. Before eruption onset, magma ascended from about 10 km depth to the surface in a few days. The melt migration occurred along the contact between consolidated oceanic crust and altered hydrothermal material at shallow depth. We postulate that similar structural discontinuities could potentially drive the formation of new eruptive centres during future eruptions.