Detection of volcanic unrest onset in La Palma, Canary Islands, evolution and implications

Abstract

La Palma island is one of the highest potential risks in the volcanic archipelago of the Canaries and therefore it is important to carry out an in-depth study to define its state of unrest. This has been accomplished through the use of satellite radar observations and an original state-of-the-art interpretation technique. Here we show the detection of the onset of volcanic unrest on La Palma island, most likely decades before a potential eruption. We study its current evolution seeing the spatial and temporal changing nature of activity at this potentially dangerous volcano at unprecedented spatial resolutions and long time scales, providing insights into the dynamic nature of the associated volcanic hazard. The geodetic techniques employed here allow tracking of the fluid migration induced by magma injection at depth and identifying the existence of dislocation sources below Cumbre Vieja volcano which could be associated with a future flank failure. Therefore they should continue being monitored using these and other techniques. The results have implications for the monitoring of steep-sided volcanoes at oceanic islands.

Figure 1

Geographical location and geological elements. Location of Canary Islands and La Palma Island (top panel) and simplified geological map (lower panel). See insets for scale and key. GMT software (www.generic-maping-tools.org) was used to create this figure.

Figure 2

Annual mean LOS velocities obtained for the La Palma Island. (A) For the period 2006–2010 results obtained with SAR images from the ENVISAT satellite acquired in ascending orbits; (B) same for images acquired in descending orbits. (C) Results obtained using images from RADARSAT-2 satellite acquired in descending orbits for the period 2010–2017. Results for 2017–2020, using Sentinel-1 images: (D) acquired in ascending orbits; and (E) in descending orbits. GMT software (www.generic-maping-tools.org) was used to create this figure.

Figure 3

Adjusted model for deformation sources. (A) Sources adjusted for the period 2006–2010, and (B) for the period 2017–2020. Each time period panel includes planar, E-W vertical and N-S vertical views of significant sources. See color code for the type of source and magnitude. Dots denote the location of ascending and descending LOS deformation data. Matlab (www.mathworks.com) and GMT (www.generic-mapin-tools.org) softwares were used to create this figure.

Figure 4

Time evolution of the modeled sources for the 2017–2020 period. The different plots show the time evolution of the different sources obtained inverting the Sentinel-1 ascending and descending LOS displacement time series every 0.6 years. Green circles show the horizontal location of the earthquakes happened during the two seismic swarms (October 2017, February 2018). See Fig. 5 for numbers of seismic events and Fig. 8 for depths of both, events and sources. Matlab software (www.mathworks.com) was used to create this figure.

Figure 5

2017–2020 average pressure time evolution for the adjusted sources. Adjusted average pressure per volume unit (blue color) and the number of seismic events (orange columns) between February 2017 and June 2020. During the October 2017 seismic swarm 122 events took place with a magnitude lower than 3 and the 95% of them with magnitude between 1 and 2. During the February 2018 seismic swarm 79 events took place with magnitude lower than 3 and the 77% of them with magnitude between 1 and 2. This figure was created using Microsoft Excel 2016.

Figure 6

Vertical sections of the 3D model for anomalous density for La Palma Island. Panels (a), (b) and (c) show E-W sections. Panels (d) and (e) N-S sections. The cross lines are indicated in the horizontal section for a depth of 800 m bsl. The horizontal section includes some interpretative lines (dotted lines). Matlab software (www.mathworks.com) was used to create this figure.

Figure 7

Horizontal sections of the 3D anomalous density structure and location of the 2006–2010 modeled pressure sources. Horizontal sections (from 2 to 10 km bsl) of the 3D model for anomalous density distribution for La Palma Island. Right upper panel shows the structure and the location of the modeled 2006–2010 positive pressure source E-W pattern compared with the structural one at 4 km and 7 km depth as a way of example. Matlab software (www.mathworks.com) was used to create this figure.

Figure 8

Conceptual model for the unrest evolution. Two main surfaces obtained in the 3D gravimetric crustal model (green and grey surfaces), the modeled positive pressure sources for the periods 2006–2010 and 2017–2020, the modeled dip-slip sources in 2019–2020, and the location of seismic events. The proposed path followed by the magma from depth aseismically (yellow path) before, and probably after, seismic swarms in 2017 and 2018 which may have opened new fractures. See lower inset for a description of the different components of the plot. Matlab software (www.mathworks.com) was used to create this figure.