Seismic Swarm S20110927.2: Analysis of Early Activity in the Canary Islands Region

The Canary Islands form a volcanic archipelago situated approximately 100 km off the northwest coast of Africa. This region originated from hotspot volcanism associated with the African plate, resulting in a chain of shield volcanoes built over millions of years. The islands exhibit a well-documented history of basaltic eruptions, with submarine and subaerial activity shaping their topography. Tenerife hosts the prominent Teide stratovolcano, while other islands such as La Palma and El Hierro have recorded multiple eruptive episodes in the Holocene. Seismic monitoring in the archipelago routinely detects low-magnitude events linked to tectonic adjustments and magmatic processes at depths typically between 5 and 25 km. Swarm S20110927.2 was recorded in the Canary Islands, Spain region. It began at 20:28 on 26 September 2011 and concluded at 20:11 on 8 October 2011, spanning 287 hours and 43 minutes. A total of 305 earthquakes were detected during this period. The first 100 events, spanning from 26 September 20:28 to 28 September 17:47, provide initial insight into swarm development. These events displayed magnitudes ranging from 1.6 to 3.8, with the majority falling between 2.5 and 3.0. The largest early event reached magnitude 3.8 at 15:44 on 27 September at 17 km depth. Depths clustered predominantly between 10 and 18 km, averaging around 14–15 km, consistent with typical mid-crustal activity in the archipelago. Early events on 26 September remained below magnitude 2.3 and occurred at depths of 9–19 km. Activity intensified on 27 September, with multiple events exceeding magnitude 3.0, including a 3.4 at 09:32 and a 3.6 at 14:38. By 28 September, the sequence maintained steady rates with repeated magnitudes of 2.7–3.3 at similar depths. This pattern indicates progressive energy release without immediate large-magnitude outliers beyond the initial phase. Event frequency increased notably after the first 12 hours, accompanied by a slight upward trend in maximum magnitudes. Depth stability suggests a consistent source volume rather than rapid vertical migration during the opening stage. Historical records since 1 January 2000 document four seismic swarms in the region. Swarm S20110927.2 represents the earliest in this sequence.

References

• Instituto Geográfico Nacional (IGN) seismic catalogs for the Canary Islands.
• Geological Survey of Spain (IGME) reports on Canary volcanism and hotspot tectonics.
• SeismoSight internal classification data for Swarm S20110927.2.