Seismic Swarm VS20210912.1 in the Canary Islands Region

The seismic swarm designated VS20210912.1 was recorded in the Canary Islands, Spain, beginning at 00:16 on 12 September 2021 and concluding at 12:22 on 23 September 2021. Over 276 hours and 5 minutes, a total of 1086 earthquakes were registered. This event represents one of only two swarms documented in the region since 2000, with the prior occurrence noted in 2020. The Canary Islands form a volcanic archipelago situated off the northwest coast of Africa, originating from the Canary hotspot beneath the African plate. The islands exhibit classic hotspot characteristics, including a linear chain of volcanoes with ages increasing eastward. Tenerife hosts the prominent Teide stratovolcano, while the western islands, particularly La Palma and El Hierro, remain the most volcanically active. Seismic swarms frequently precede eruptive episodes due to magma intrusion and associated fracturing in the crust. Depths of events in this swarm predominantly ranged between 5 and 19 km, consistent with shallow crustal processes typical of the archipelago's volcanic systems. Analysis of the first 100 events reveals a rapid onset with initial magnitudes of 1.5 at depths of 19 km and 17 km. Magnitudes escalated quickly, reaching a peak of 2.8 within the first day, with multiple events at or above 2.5. Depths shallowed progressively in many cases, clustering around 8–12 km as activity intensified. Temporal distribution showed dense sequences, including several events within minutes during afternoon hours on 12 September, indicative of sustained stress release. The majority of these early quakes maintained magnitudes between 1.5 and 2.8, reflecting low-to-moderate energy release characteristic of swarm behavior rather than a single mainshock-aftershock sequence. Geological records confirm that such swarms often signal magma movement at depth, as observed in prior activity on El Hierro in 2011–2012 and La Palma in 2021. The 2021 swarm aligns with this pattern, occurring amid heightened monitoring of the Cumbre Vieja ridge on La Palma. Historical data since 2000 underscore the rarity of these episodes, highlighting their significance for volcanic hazard assessment in the region. This swarm underscores the dynamic nature of Canary Islands volcanism, where recurrent seismic unrest necessitates continuous geophysical surveillance to mitigate potential impacts on local populations and infrastructure.

References

• Instituto Geográfico Nacional (IGN) seismic catalogs for Canary Islands.
• Geological Survey of Spain (IGME) volcanic history reports.
• Peer-reviewed studies on Canary hotspot dynamics in Journal of Volcanology and Geothermal Research.