Location:

Period:

12 Sep 2001 12:50:16 - 14 Sep 2001 04:45:08 (1 day 15 hours 54 minutes)

Volcanoes in 100km radius:

West Valley Segment(8km)

Earthquakes:

2 swarms found nearby.

2004

PS20041102.1(72.3km)

2 Nov

7 hours

6 earthquakes

2008

PS20080828.1(172.0km)

27 Aug

5 hours

5 earthquakes

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Seismic Swarm PS20010913.1: Earthquake Sequence West of Tofino, Canada

Seismic swarm PS20010913.1 was recorded 155 km west of Tofino, Canada, beginning at 12:50 on 12 September 2001 and concluding at 04:45 on 14 September 2001. Over this 39-hour, 54-minute interval, six earthquakes occurred, providing a clear example of swarm behavior in an active subduction setting.

The sequence unfolded as follows. The initial event registered magnitude 5.1 at 12:50:16 on 12 September at a depth of 10 km. Roughly nine hours later, at 22:24:54, a magnitude 5.3 earthquake struck at the same depth, followed minutes afterward at 22:30:21 by a magnitude 5.2 event. On 13 September, two additional shocks occurred: a magnitude 5.1 at 09:56:58 and a smaller magnitude 3.4 at 10:04:45, both at 10 km depth. The swarm culminated with a magnitude 6.0 event at 04:45:08 on 14 September, again at 10 km depth.

This region forms part of the Cascadia subduction zone, where the Juan de Fuca plate converges with and descends beneath the North American plate at rates of approximately 4 cm per year. The shallow focal depths of all events align with activity near the plate interface or within the overriding crust. Historical records document recurrent megathrust earthquakes in Cascadia, most notably the magnitude 9-class event of 26 January 1700, which generated a trans-Pacific tsunami. Modern instrumentation has since identified numerous earthquake swarms along the margin, often linked to episodic tremor and slip processes.

Swarm activity such as PS20010913.1 typically lacks a dominant mainshock-aftershock pattern and instead reflects distributed stress release. In subduction environments, such sequences may arise from fluid migration along the plate boundary or from transient changes in frictional properties. The consistent 10 km depths and moderate magnitudes observed here are consistent with these mechanisms and underscore the value of continuous monitoring for assessing short-term seismic hazards in the offshore Cascadia region.

References

SeismoSight internal classification records for swarm PS20010913.1.
USGS Earthquake Hazards Program summaries of Cascadia seismicity.
Natural Resources Canada Geological Survey reports on Vancouver Island tectonics.