Earthquake Swarm S20241126.1: Seismic Activity Near the West Coast of Honshu

A seismic swarm designated S20241126.1 was recorded near the west coast of Honshu, Japan. The event sequence began at 13:47 on 26 November 2024 and ended at 09:22 on 29 November 2024. Over 67 hours and 35 minutes, a total of 54 earthquakes were detected.

The swarm opened with a magnitude 6.1 earthquake at 8 km depth. This was followed by a series of aftershocks, predominantly in the 3.5–4.9 range and concentrated at depths of approximately 10 km. Notable events included multiple magnitude 4.2–4.9 shocks on 26 November, with additional magnitude 4.3 events occurring on 27 November and a magnitude 4.5 shock on 28 November. Activity tapered toward the close of the swarm, ending with a magnitude 4.0 event at 9:22 on 29 November.

This swarm illustrates typical aftershock behavior following a moderate mainshock, with rapid decay in both frequency and magnitude after the initial hours. Depths remained shallow throughout, consistent with crustal faulting in the region. The pattern suggests localized stress release along pre-existing faults without escalation to a larger main event.

The west coast of Honshu lies within a tectonically complex zone where the Eurasian Plate interacts with the subducting Pacific Plate. This setting produces frequent shallow crustal earthquakes along active fault systems. The area has a documented history of seismic sequences, including the destructive 2004 Niigata-Chuetsu earthquake and the January 2024 Noto Peninsula sequence, both of which involved similar shallow-focus activity.

Records since 2000 indicate only one prior swarm in the immediate vicinity, which occurred earlier in 2024. The current swarm therefore represents a rare clustered episode in the modern instrumental era for this specific coastal segment.

References

• SeismoSight internal classification and event catalog
• Japan Meteorological Agency seismic bulletins
• Geological Survey of Japan tectonic summaries