Seismic Swarm PS20191223.1: Analysis of Earthquake Activity West of Port McNeill

On 23 December 2019, a seismic swarm designated PS20191223.1 was recorded approximately 147 km west of Port McNeill, Canada. The sequence began at 16:44 UTC and concluded at 22:50 UTC, spanning six hours and six minutes. During this period, five earthquakes were registered, all occurring at a focal depth of 10 km.

The events unfolded in rapid succession. The initial shock at 16:44:13 measured magnitude 5.2. This was followed at 19:13:25 by a magnitude 5.7 event. Two magnitude 6.0 earthquakes occurred at 19:49:43 and 20:56:23, representing the peak intensities of the swarm. The sequence ended with a magnitude 4.1 event at 22:50:17. Such clustered activity without a clear dominant mainshock-aftershock progression is characteristic of swarm behavior in this offshore region.

The location lies within a tectonically complex zone offshore British Columbia, influenced by interactions among the Pacific, Explorer, and North American plates. This setting features transform faults, spreading ridges, and subduction-related structures that accommodate plate motion. Earthquake swarms in this area often reflect episodic stress release along these boundaries rather than single large ruptures.

Historical records since 1 January 2000 indicate five prior swarms in the same region. These occurred in 2004 (one swarm), 2008 (three swarms), and 2013 (one swarm). The 2019 sequence aligns with this pattern of intermittent, short-duration clusters.

Seismic monitoring of such swarms contributes to understanding regional strain accumulation and informs hazard assessments for coastal communities. Continued observation remains essential given the area's ongoing tectonic activity.

References

SeismoSight internal swarm classification records for PS20191223.1.
Geological Survey of Canada tectonic summaries for the northern Cascadia margin.