Early warning system for monitoring landslides of pyroclast and lahars from the 2021 eruption of the Tajogaite volcano on the island of La Palma, Canary Islands, Spain

The eruption of the Tajogaite volcano on the island of La Palma lasted for 85 days, between September and December 2021. Large quantities of lava and pyroclasts were emitted, exceeding 200 Mm³. It has been estimated that 45 Mm3 corresponded to pyroclasts.

The stability conditions of the new Tajogaite volcanic edifice were analysed to determine its hazard potential. The volcanic cone is composed of pyroclastic materials, mainly lapilli (2-64 mm) and scoria (> 64 mm), with intercalated layers of ash (< 2 mm) and encrusted sulphate and carbonate precipitates. The estimated height of the cone reaches 200 m, with slopes of 30-35º, which have fractures that favour the emission of gases. The stability analysis under unsaturated conditions yielded a safety factor of 1.2, which in geotechnical terms is equivalent to stable conditions; however, under saturated conditions, the safety factor is less than 1.00, indicating instability or failure under very heavy rainfall.

According to rainfall records and historical data, this region could experience heavy rainfall of more than 100 mm in several hours, with a possible frequency of once every 10 years, and exceptionally, accumulated rainfall of more than 400 mm could occur over several days in a 50-year interval. If these conditions occur, the pyroclastic materials of the cone may become saturated and unstable, and lahars may occur.

Given the risk of lahars, whose probability is low or very low, and the instability of the volcanic cone slopes, this volcano has been included as a study area within the PRISMAC project, which plans to establish an early warning system for landslides using geospatial technologies.

The PRISMAC project (1/MAC/2/2.4/0112), co-financed by the INTERREG VI D Madeira-Azores-Canary Islands MAC 2021-2027 Territorial Cooperation Program, aims primarily to analyze, mitigate, and manage natural hazards, with a particular focus on landslide movements, which are increased by the effects of climate change. To achieve this, harmonized methodologies for susceptibility and risk analysis are being developed, enabling the identification of high-risk areas within the participating Macaronesian regions. This will facilitate the creation of monitoring systems, early warning, and alarm mechanisms, which are essential for reducing the impact of these phenomena on populations and infrastructure.

The early warning and alarm system proposed by PRISMAC is based on the development of algorithms that take into account critical rainfall thresholds, combined with aerial geospatial techniques (drones with LiDAR systems), terrestrial techniques (high-precision 3D laser scanners) and satellites (Sentinel-1 radar using the InSAR technique to detect millimetric ground movements).