Dike intrusion before the 2021 La Palma eruption

On 19 September 2021 at 14:10h (UTC) a volcanic eruption started in the South of La Palma (Canary Islands). Just a week earlier, on 11 September, an intense seismic swarm had begun in the area, with the hypocenters located at 11 km depth in the first days but gradually approaching the surface throughout the week. This activity, together with an evident deformation recorded both at the island's GNSS stations and in InSAR measurements, were key to the monitoring of the reactivation and estimate the eruption onset parameters. 

In this presentation we show the results obtained using the volcanic monitoring network of the Instituto Geográfico Nacional (IGN) before La Palma eruption. Consistent results have been obtained combining seismic and geodetic techniques. We have used hypoDD relative location algorithm to improve the hypocenters of 1323 earthquakes of the IGN catalog. Deformation results have been obtained using Sentinel-1 images to get the InSAR interferograms and GNSS time series have been computed using double differences with Bernese software considering a regional network.  

At the beginning, seismicity was 11 kilometers below the central part of Cumbre Vieja and for ~6 days it migrated towards the surface in northwest direction. Meanwhile, the deformation indicated a magma intrusion in the area of the activity, showing a good correlation with seismic data. Some hours before the eruption started there was a sudden change in the migration direction pointing north while hypocenters considerably accelerated its trend to the surface. This episode was accompanied by a rapid deformation of more than 7 cm to the South and 5 cm upwards in the closest GNSS station. Finally, 4 hours before the eruption, an increase in the shallow seismicity rates was observed. Most of the earthquakes were not felt by the island population and moderate magnitudes were recorded reaching a maximum of 3.8 (mbLg). InSAR results during the whole process show more than 20 cm of deformation in LOS (Line of Sight) to the South of the eruption vent. 

Hours before the eruption, the seismicity behavior and the deformation shape indicate the existence of an intruding dike that culminated in eruption and that would be the last stage of a magmatic process that had begun at least four years earlier with the seismic reactivations on the island.