Seismic Swarm PS20130904.1: Analysis of Offshore Activity West of Vancouver Island

A seismic swarm designated PS20130904.1 was recorded 222 km west-northwest of Port McNeill, British Columbia. The sequence began at 20:19 on 3 September 2013 and concluded at 00:50 on 4 September 2013, lasting 4 hours and 31 minutes. Five earthquakes were detected during this interval, with magnitudes ranging from 4.6 to 6.1 and focal depths between 2 km and 10 km.

The events unfolded in rapid succession. The initial shock at 20:19:06 reached magnitude 6.1 at a depth of 2 km. A magnitude 5.1 event followed at 21:25:16 at 10 km depth. The third quake, magnitude 5.5, occurred at 22:29:31 at 9 km depth. At 00:23:11 on 4 September a magnitude 6.0 shock was recorded at 9 km depth. The swarm ended with a magnitude 4.6 event at 00:50:59 at 10 km depth.

Earthquake swarms in this offshore region typically reflect transient stress adjustments along plate boundaries rather than foreshock-mainshock-aftershock sequences. The clustered timing and moderate magnitudes observed here align with patterns seen in prior swarms recorded since 2000, which total four episodes. Earlier swarms occurred in 2008 (two events) and 2012 (two events). Such activity is consistent with the tectonic regime offshore northern Vancouver Island, where the Explorer Plate interacts with the Pacific Plate and the North American Plate.

Geologically, the location lies within the northern extent of the Cascadia subduction zone. Here the young, buoyant Explorer Plate subducts beneath North America at a shallow angle, producing a broad zone of seismicity. The region also features transform faulting and short spreading segments that accommodate relative motion between the Explorer and Pacific plates. These structures generate frequent small-to-moderate earthquakes, occasional swarms, and, over longer timescales, the potential for great megathrust events. Historical records and paleoseismic evidence indicate that the Cascadia margin has hosted magnitude-9-class earthquakes at intervals of several centuries, with the most recent great event dated to 1700.

Depths in the 2–10 km range for the 2013 swarm place the hypocenters within the upper crust or near the plate interface, where frictional properties can favor swarm-like behavior through fluid migration or aseismic slip transients. The absence of a single dominant mainshock and the tight temporal clustering support classification as a swarm rather than a classic aftershock sequence.

Ongoing monitoring by regional seismic networks continues to track microseismicity in this area. Understanding swarm characteristics contributes to improved models of stress transfer and helps refine hazard assessments for coastal British Columbia communities.

References

• Geological Survey of Canada, Earthquake Database (NRCan)
• USGS Earthquake Catalog and Tectonic Summaries
• Paleoseismic studies of the Cascadia subduction zone (Atwater et al., various publications)