Seismic Swarm S20110808.1 in the Canary Islands Region

Seismic swarm S20110808.1 occurred in the Canary Islands, Spain region, beginning at 06:53 on 7 August 2011 and concluding at 15:40 on 10 August 2011. Over this 80-hour and 47-minute period, a total of 64 earthquakes were recorded. The events exhibited magnitudes ranging from 0.6 to 2.5, with the majority falling between 1.4 and 2.2. Focal depths clustered predominantly between 8 and 15 km, consistent with shallow crustal activity in a volcanic setting.

The sequence initiated with events of magnitude 1.9 and 1.7 on the first day, followed by a cluster of 1.6–2.0 magnitude shocks concentrated in the evening hours. Activity continued into 8 August with lower-magnitude events interspersed among slightly stronger ones reaching 2.1. By 9 August, the swarm intensified, producing multiple events above magnitude 2.0, including a peak of 2.5 at 15:52. The final day featured a steady decline, with events tapering to magnitude 1.8 by the swarm’s end. Depths remained stable throughout, rarely deviating beyond the 10–13 km range.

This swarm represents the first of two recorded since 1 January 2000, highlighting episodic seismic unrest in the archipelago. Such sequences often reflect fluid migration or stress adjustments within the volcanic system rather than isolated tectonic rupture. The tight clustering of depths and moderate magnitudes aligns with patterns observed in basaltic volcanic provinces, where magma movement can trigger prolonged microseismicity without immediate surface 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 the Canary hotspot, which has generated a chain of shield volcanoes over millions of years. The islands exhibit classic features of intraplate basaltic volcanism, including extensive lava flows, cinder cones, and caldera structures. Tenerife, La Palma, and El Hierro host the most recent Holocene activity, underscoring the region’s ongoing volcanic potential.

Seismic monitoring in the Canary Islands has documented recurrent low-magnitude swarms linked to magmatic processes beneath the volcanic edifices. Depths of 8–15 km correspond to the brittle-ductile transition zone where ascending magma or hydrothermal fluids interact with the crust. Historical records indicate that similar swarms preceded the 2011 submarine eruption at El Hierro, illustrating the value of swarm detection for eruption forecasting.

Geological mapping and geophysical surveys confirm that the archipelago rests on Jurassic oceanic crust thickened by hotspot-related underplating. The islands’ stratigraphy comprises alternating layers of basaltic lavas, pyroclastic deposits, and sedimentary interbeds. Active fault systems, often radial or concentric around volcanic centers, accommodate both tectonic and volcanic deformation.

Continued surveillance by regional networks remains essential given the archipelago’s population density and tourism infrastructure. The 2011 swarm provided an early example of how dense microseismic sequences can signal subsurface magmatic reorganization in this geologically active province.

References

SeismoSight internal swarm catalogue
Instituto Geográfico Nacional (IGN) seismic bulletins
Geological Survey of Spain (IGME) volcanic hazard assessments