Location:

Period:

27 Aug 2008 13:35:28 - 28 Aug 2008 16:20:41 (1 day 2 hours 45 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

3 swarms found nearby.

2004

PS20041102.1(105.3km)

2 Nov

7 hours

6 earthquakes

2008

PS20080828.1(6.6km)

27 Aug

5 hours

5 earthquakes

2019

PS20191223.1(37.4km)

23 Dec

6 hours

5 earthquakes

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Seismic Swarm S20080828.1 Off Port McNeill, Canada

Seismic swarm S20080828.1 occurred approximately 192 km west-southwest of Port McNeill, British Columbia, Canada. The sequence began at 13:35 UTC on 27 August 2008 and concluded at 16:20 UTC on 28 August 2008, lasting 26 hours and 45 minutes. During this period, 36 earthquakes were recorded, with the majority occurring at depths of 10–12 km.

The swarm featured several events exceeding magnitude 4.0. Notable shocks included a magnitude 4.8 at 21:52 on 27 August, another magnitude 4.8 at 04:47 on 28 August, and a magnitude 5.0 at 09:20 on 28 August. The largest event reached magnitude 5.9 at 12:37 on 28 August at a depth of 12 km. Smaller events, including those below magnitude 4.0, clustered primarily in the later stages of the sequence.

This region lies within the northern segment of the Cascadia subduction zone, where the Explorer Plate interacts with the North American Plate. The tectonic setting involves oblique subduction and transform faulting along boundaries such as the Nootka Fault and the Explorer Ridge. Earthquake swarms in this area commonly result from fluid migration or aseismic slip along pre-existing faults, releasing accumulated stress without producing a single dominant mainshock.

Historical records indicate limited swarm activity in the vicinity since 2000. The preceding swarm in this classification occurred in 2004, highlighting the episodic nature of seismic clusters along the plate boundary. Such events contribute to the overall seismic hazard assessment for coastal British Columbia, where monitoring supports tsunami and earthquake preparedness.

The 2008 swarm aligns with patterns observed in subduction-related environments, where moderate-magnitude sequences can occur without significant surface rupture. Depths remained shallow, consistent with crustal deformation above the subducting slab. No damage or felt reports were associated with these offshore events.

References

Natural Resources Canada Earthquake Database
Geological Survey of Canada tectonic reports on the Explorer Plate
USGS Earthquake Catalog for regional events