Seismic Swarm PS20061120.1 in the Kuril Islands: November 2006 Analysis

The Kuril Islands form a volcanic arc along the convergent boundary where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting produces frequent seismicity, including both large megathrust earthquakes and episodic earthquake swarms. The region has a well-documented history of great earthquakes, with recurrence intervals for magnitude 8+ events on the order of decades to centuries, driven by the accumulation and release of strain along the subduction interface.

Swarm PS20061120.1 was recorded between 15:16 UTC on 19 November 2006 and 16:48 UTC on 20 November 2006. Over this 25-hour-and-31-minute period, seven earthquakes were detected. The sequence comprised events with the following parameters: a magnitude 5.5 earthquake at 10 km depth on 19 November at 15:16:52; a magnitude 5.0 event at 30 km depth at 21:11:28; a magnitude 5.1 event at 10 km depth at 21:53:20; another magnitude 5.1 at 10 km depth on 20 November at 00:45:39; a magnitude 4.6 at 10 km depth at 05:04:36; a magnitude 5.1 at 10 km depth at 05:27:28; and a final magnitude 5.0 at 10 km depth at 16:48:26. Depths were predominantly shallow, consistent with activity near the plate interface or within the overriding crust.

Earthquake swarms differ from typical aftershock sequences in that they lack a single dominant mainshock and instead exhibit clustered activity driven by fluid migration, stress transfer, or aseismic slip. In subduction zones such as the Kurils, swarms often occur in the forearc or near volcanic centers and may signal transient changes in pore pressure or slip behavior along the megathrust. The timing of this swarm, beginning four days after the 15 November 2006 magnitude 8.3 Kuril Islands earthquake located 81 km from the swarm centroid, suggests a possible link through static or dynamic stress changes induced by the mainshock.

Since 1 January 2000, eight swarms have been identified in the Kuril Islands according to internal classification records. Prior occurrences were limited to single swarms in 2003 and 2005, with six additional swarms documented in 2006 alone, indicating a period of elevated swarm activity that year. Such clustering may reflect broader changes in regional stress or fluid regimes following major subduction-zone ruptures.

The 15 November 2006 magnitude 8.3 event itself was one of the largest instrumentally recorded earthquakes in the Kuril arc during the preceding decades. It ruptured a segment of the megathrust and generated a modest tsunami, underscoring the persistent seismic hazard posed by the plate boundary. Ongoing monitoring in the region continues to track both isolated events and swarm activity to improve understanding of subduction dynamics and to refine probabilistic hazard assessments.

References

• USGS Earthquake Catalog (https://earthquake.usgs.gov)
• SeismoSight internal swarm classification records