Seismic Swarm PS20181011.1 Near Severo-Kuril’sk: Insights into Kuril Subduction Zone Activity

The Kuril Islands form part of the tectonically active Kuril-Kamchatka arc, where the Pacific Plate subducts beneath the Okhotsk Plate at rates exceeding 8 cm per year. This convergent boundary generates frequent seismicity, including deep-focus events and occasional shallow swarms linked to stress transfer along the megathrust and overlying crustal faults. Severo-Kuril’sk, located on Paramushir Island, lies approximately 138 km north of the swarm epicenter, placing the sequence within a region of known volcanic and seismic hazards associated with the arc’s ongoing subduction dynamics.

Swarm PS20181011.1 began at 23:16 UTC on 10 October 2018 and concluded at 15:08 UTC on 11 October 2018, spanning 15 hours and 52 minutes. During this interval, nine earthquakes were recorded. The sequence initiated with a magnitude 6.5 event at 20 km depth, followed by eight additional shocks with magnitudes ranging from 4.7 to 5.5, predominantly at depths of 10 km. The final event measured magnitude 5.1 at 17 km depth. Such swarms typically reflect fluid migration or aseismic slip episodes that trigger clustered brittle failure without a dominant mainshock-aftershock pattern.

Event timings and parameters are as follows: the magnitude 6.5 shock at 23:16:02 on 10 October; a magnitude 5.2 event at 23:19:42; magnitude 5.5 at 23:21:50; magnitude 4.7 at 00:14:18 on 11 October; magnitude 5.4 at 00:14:39; magnitude 5.0 at 01:30:20; magnitude 5.2 at 02:16:16; magnitude 5.2 at 05:47:32; and the closing magnitude 5.1 shock at 15:08:10. Depths remained shallow to intermediate, consistent with upper-plate or interface processes in the subduction system.

Historical records since 2000 indicate only three prior swarms in the immediate vicinity: single-swarm episodes in 2004, 2008, and 2012. This low recurrence suggests that swarm-type activity is infrequent relative to the region’s background seismicity, which includes great earthquakes exceeding magnitude 8. The 2018 sequence therefore provides a valuable window into transient deformation within an otherwise locked portion of the megathrust.

Geological context underscores the arc’s long-term evolution. Subduction initiated in the Cretaceous, building a chain of volcanic islands and back-arc basins. Modern monitoring networks detect both interplate thrust events and intraslab normal-faulting earthquakes, with the latter often occurring at depths greater than 100 km. Shallow swarms such as PS20181011.1 may indicate episodic unlocking of subsidiary faults above the plate interface, potentially modulated by nearby volcanic systems including those on Paramushir and adjacent islands.

In summary, the 2018 swarm highlights the dynamic interplay between subduction-driven stress accumulation and short-term release mechanisms in the southern Kuril segment. Continued observation of similar sequences will refine models of seismic hazard for populations and infrastructure along the arc.

References: SeismoSight internal swarm classification PS20181011.1 USGS Earthquake Catalog (regional tectonic framework) Global CMT Project (subduction zone focal mechanisms)