Seismic Swarm Near West Coast of Honshu, Japan: Analysis of January 2024 Event

A seismic swarm designated S20240101.2 occurred near the west coast of Honshu, Japan, from 07:32 on 1 January 2024 to 20:51 on 3 January 2024. Over 61 hours and 18 minutes, 48 earthquakes were recorded, with magnitudes ranging from 3.5 to 5.3 and focal depths primarily between 0 and 20 km. The sequence began with a 4.4-magnitude event at 13 km depth and included multiple events above magnitude 4.0, culminating in a 5.3-magnitude earthquake at 10 km depth on 3 January.

This swarm unfolded in a tectonically active region where the Eurasian Plate interacts with the Pacific and Philippine Sea plates. The west coast of Honshu lies along the eastern margin of the Sea of Japan, influenced by back-arc spreading and strike-slip faulting within the Niigata-Kobe Tectonic Zone. Shallow crustal earthquakes are common here due to compressional stresses transmitted from the subduction zones to the east and south. Depths recorded in the swarm align with typical crustal seismicity in this area, where brittle failure occurs above approximately 20 km.

Historical records indicate limited swarm-type activity in the immediate vicinity since 2000, with only one prior swarm documented in 2024. The January 2024 sequence represents clustered seismicity without a single dominant mainshock, consistent with fluid migration or stress triggering along local faults. Magnitudes remained moderate, and no events exceeded 5.3, reflecting the region's capacity for both isolated larger shocks and episodic swarms.

Geological context underscores ongoing deformation. The Sea of Japan formed through Miocene extension, leaving inherited structures that accommodate present-day convergence. Monitoring by agencies such as the Japan Meteorological Agency confirms that such swarms contribute to the broader seismic hazard assessment for coastal Honshu communities.

References

• Japan Meteorological Agency seismic catalogs (2024 updates)
• USGS Earthquake Hazards Program: Tectonic summary for Japan
• Geological Survey of Japan: Active fault database for Honshu west coast