Seismic Swarm VS20110913.1: Insights from the Canary Islands Region

The Canary Islands, an archipelago of volcanic origin located approximately 100 kilometers off the northwest coast of Africa, form part of Spain and lie on the African tectonic plate. Their geology is shaped by a mantle hotspot, resulting in shield volcanoes and basaltic lava flows. The islands feature prominent stratovolcanoes such as Mount Teide on Tenerife, which rises to 3,718 meters and remains active. Seismic activity in the region is primarily linked to volcanic processes, including magma movement and associated fracturing, rather than large-scale tectonic subduction.

Historical records indicate that the Canary Islands experience periodic seismic swarms tied to volcanic unrest. Notable events include the 2011-2012 El Hierro eruption, preceded by thousands of earthquakes, and the 2021 Cumbre Vieja eruption on La Palma, which involved extensive seismic monitoring. These swarms typically consist of low-magnitude events clustered in time and space, reflecting fluid migration or pressure changes beneath the surface. Updated geological assessments confirm ongoing monitoring by Spanish institutions due to the archipelago's volcanic hazard potential.

SeismoSight internal classification identifies swarm VS20110913.1 as the first such event recorded since January 1, 2000, occurring in the Canary Islands, Spain region. The swarm began at 03:03 on September 12, 2011, and concluded at 13:08 on September 13, 2011, spanning 34 hours and 4 minutes with a total of 32 earthquakes. Magnitudes ranged from 1.1 to 2.7, with depths predominantly between 9 and 12 kilometers, suggesting shallow crustal activity consistent with volcanic systems.

Event timing shows clustering in the early hours of September 12, followed by sporadic activity through the night and into September 13. The largest event reached magnitude 2.7 at 06:48 on September 12 at a depth of 9 kilometers. Multiple events of magnitude 2.4 to 2.6 occurred at similar depths around 9-11 kilometers, indicating possible localized stress release along volcanic conduits. Shallower events at 9 kilometers were more frequent in the initial phase, while slightly deeper occurrences at 12 kilometers appeared later in the sequence.

This swarm provides insight into precursory volcanic seismicity patterns. The tight spatial and temporal grouping, combined with consistent shallow depths, aligns with magma-induced fracturing rather than tectonic faulting. No larger mainshock-aftershock sequence was observed, reinforcing the swarm classification. Such episodes often precede or accompany volcanic inflation, underscoring the importance of real-time seismic networks for hazard assessment in the Canary Islands.

Further analysis of depth distribution reveals a concentration around 9-11 kilometers, potentially corresponding to the brittle-ductile transition zone influenced by geothermal gradients in the hotspot setting. The absence of events exceeding magnitude 3.0 suggests limited energy release, typical of non-eruptive swarms in the region.

References

• SeismoSight internal swarm classification data for VS20110913.1
• Instituto Geográfico Nacional (IGN) volcanic monitoring reports on Canary Islands geology
• Geological Survey of Spain (IGME) updates on hotspot volcanism and seismic hazards