Seismic Swarm VS20110723.1: Analysis of Activity in the Canary Islands Region

A seismic swarm designated VS20110723.1 was recorded in the Canary Islands, Spain region, commencing at 09:36 on 22 July 2011 and concluding at 07:31 on 23 July 2011. Over this 21-hour 55-minute interval, 25 earthquakes were registered, providing insight into localized tectonic and volcanic processes.

The sequence featured events with magnitudes ranging from 1.3 to 2.3 and focal depths between 6 and 12 km. Early activity included a magnitude 1.7 event at 11 km depth, followed by several magnitude 2.0 shocks at similar depths. Peak activity occurred in the late evening of 22 July, with multiple events above magnitude 2.0 clustered between 20:25 and 23:00. The swarm continued into the morning of 23 July with consistent low-magnitude releases before tapering off. Such patterns are characteristic of volcanic swarms, where rapid successions of small events often reflect fluid migration or stress adjustments within the crust rather than a single large rupture.

The Canary Islands form a volcanic archipelago situated on the African tectonic plate, approximately 100 km off the northwest coast of Africa. Their origin is attributed to a mantle hotspot beneath the slowly moving plate, producing a chain of shield volcanoes. The islands exhibit ongoing volcanic and seismic activity driven by both hotspot-related magmatism and regional tectonics associated with the Atlas Mountains. Depths of 6–12 km align with typical upper-crustal seismicity in this setting, where magma reservoirs and hydrothermal systems interact with brittle rock layers.

Historically, the Canary Islands have experienced recurrent seismic swarms linked to volcanic unrest. Notable episodes include pre-eruptive activity at La Palma in 1949 and 1971, as well as the 2011–2012 unrest at El Hierro, which featured thousands of events prior to submarine eruption. Monitoring by Spain’s Instituto Geográfico Nacional (IGN) has documented how swarms in this magnitude range frequently precede or accompany intrusive episodes without culminating in surface eruptions. Updated geological assessments confirm the archipelago remains one of Europe’s most active volcanic provinces, with continuous microseismicity reflecting persistent magmatic dynamics.

This particular swarm offers evidence of short-term crustal adjustment beneath the region. The tight clustering of depths and moderate magnitudes suggests a compact source volume, possibly associated with dyke propagation or pressure changes in a shallow reservoir. No larger mainshock occurred, consistent with swarm behavior in volcanic environments where energy is released through numerous small failures.

References

• Instituto Geográfico Nacional (IGN) seismic catalogs and volcanic monitoring reports.
• Geological Survey of Spain (IGME) publications on Canary Islands volcanism and tectonics.
• Peer-reviewed literature on hotspot volcanism and seismic swarms in oceanic islands.