Location:

Period:

2 Nov 2004 09:30:47 - 2 Nov 2004 17:23:42 (7 hours 52 minutes)

Volcanoes in 100km radius:

West Valley Segment(70km)

Earthquakes:

4 swarms found nearby.

2001

PS20010913.1(72.3km)

12 Sep

1 day 15 hours

6 earthquakes

2008

S20080828.1(105.3km)

27 Aug

1 day 2 hours

36 earthquakes

PS20080828.1(102.6km)

27 Aug

5 hours

5 earthquakes

2019

PS20191223.1(133.3km)

23 Dec

6 hours

5 earthquakes

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Seismic Swarm PS20041102.1: The November 2004 Sequence Southwest of Port McNeill

On 2 November 2004, a seismic swarm designated PS20041102.1 was recorded 206 km southwest of Port McNeill, Canada. The sequence began at 09:30 and concluded at 17:23, comprising six earthquakes within a span of seven hours and fifty-two minutes. All events occurred at a focal depth of 10 km, with magnitudes ranging from 5.1 to 6.7.

The largest event reached magnitude 6.7 at 10:02:12, followed by five additional shocks of magnitudes 5.1 to 5.2. The precise timing of the events is as follows: 09:30:47 (M5.1), 10:02:12 (M6.7), 11:47:11 (M5.2), 13:11:05 (M5.2), 14:03:44 (M5.1), and 17:23:42 (M5.2). This tightly clustered activity is characteristic of swarm behavior, in which multiple events occur without a single dominant mainshock-aftershock pattern.

The location lies within the tectonically active offshore region of northern Vancouver Island, part of the complex boundary between the North American, Pacific, and Explorer plates. This area features transform faults, spreading ridges, and subduction-related structures associated with the northern termination of the Cascadia subduction zone. Historical seismicity in the region reflects ongoing plate interactions, including strike-slip motion along the Queen Charlotte Fault system and deformation near the Explorer Plate.

Since 1 January 2000, only one prior swarm has been documented in this specific classification, occurring in 2001. The 2004 sequence therefore represents a rare recorded instance of swarm activity in the monitored record for the site. Such events provide insight into episodic strain release along secondary faults or volcanic systems in the broader tectonic framework.

Geological studies of the northern British Columbia margin emphasize the influence of the Explorer Ridge and associated fracture zones on local seismicity. Earthquakes in this setting commonly occur at shallow to intermediate depths, consistent with the 10 km focal depths observed in the swarm. The absence of significant aftershock migration or escalation beyond the recorded magnitudes aligns with typical swarm dynamics in oceanic transform environments.

Further monitoring of this offshore domain contributes to understanding regional seismic hazards and plate-boundary processes. Continued observation may clarify whether similar swarms recur in association with specific tectonic features southwest of Port McNeill.

References:

SeismoSight internal swarm classification records
Geological Survey of Canada tectonic summaries for the northern Cascadia margin
Natural Resources Canada earthquake database (historical context)