Seismic Swarm S20230914.1 on Vancouver Island, Canada

A seismic swarm designated S20230914.1 occurred in the Vancouver Island region of Canada from 23:38 UTC on 13 September 2023 to 21:18 UTC on 18 September 2023. Over 117 hours and 40 minutes, 74 earthquakes were recorded. All events were located at a consistent depth of 10 km, with magnitudes ranging from 2.6 to 4.2. The swarm featured multiple events above magnitude 3.5, including peaks of 4.2 on 14 September at 17:31 and 17:41 UTC, and additional magnitude 4.0 events on 14 and 15 September.

The sequence began with a magnitude 2.8 event and rapidly intensified within the first 24 hours, producing the majority of higher-magnitude shocks. Activity then declined gradually, with the final recorded event measuring magnitude 3.2. No single mainshock dominated the sequence; instead, the events clustered temporally without clear foreshock-aftershock progression, consistent with swarm characteristics.

Vancouver Island lies within the northern segment of the Cascadia subduction zone, where the Juan de Fuca plate converges with and subducts beneath the North American plate at approximately 4 cm per year. This tectonic setting produces frequent seismicity, including both interface thrust events and intraslab earthquakes within the subducting plate. The swarm’s uniform 10 km depth aligns with typical intraslab activity in this region. Historical great earthquakes, such as the magnitude 9.0 event of 1700, demonstrate the zone’s capacity for megathrust ruptures, while smaller swarms reflect localized stress adjustments or fluid migration along faults.

Since 2000, four swarms have been documented in the area according to internal SeismoSight classification: two in 2008, one in 2013, and one in 2019. The 2023 swarm represents the most recent episode. Such recurrent swarms indicate episodic seismic release without progression toward larger events. Magnitudes remained below levels likely to cause significant damage, though felt shaking was probable across parts of Vancouver Island and the adjacent mainland.

Ongoing monitoring by regional seismic networks continues to track activity in this tectonically active margin. Swarms like S20230914.1 provide valuable data on subduction-zone dynamics and contribute to refined hazard assessments for southwestern British Columbia.

References
SeismoSight internal swarm classification S20230914.1
Natural Resources Canada Earthquake Database
USGS Earthquake Catalog
Cascadia Subduction Zone tectonic summaries from Geological Survey of Canada