Seismic Swarm S20240101.3 Near the West Coast of Honshu, Japan

Seismic swarm S20240101.3 was recorded along the west coast of Honshu, Japan, beginning at 07:27 on 1 January 2024 and concluding at 10:49 on 3 January 2024. Over 51 hours and 22 minutes, a total of 36 earthquakes were detected. The events ranged in magnitude from 3.5 to 4.9, with the majority occurring at depths of approximately 10 km. The initial sequence on 1 January featured several events above magnitude 4.5 within the first few hours, including peaks at 4.8 and 4.9. Activity continued at a moderate pace through 2 January, with additional notable shocks at 4.8 and 4.4, before tapering on 3 January with four final events between 4.1 and 4.2.

This swarm represents a concentrated release of seismic energy in a short period, characteristic of episodic fault slip rather than a single mainshock-aftershock sequence. Most events clustered at shallow crustal depths, suggesting activation of local fault segments within the upper 20 km of the crust. The temporal distribution shows an initial high-frequency burst followed by a gradual decline, consistent with fluid migration or stress transfer along pre-existing fractures in the region.

The west coast of Honshu occupies a tectonically complex zone influenced by the convergence of the Eurasian, Pacific, and Philippine Sea plates. This setting produces both subduction-related and intraplate seismicity, with active fault systems accommodating east-west compression. The area has a documented history of moderate earthquake swarms linked to these structures, though only two such swarms have been identified since 2000, occurring in 2023 and 2024.

Geological records indicate that the region experiences recurrent seismic episodes due to its position near the eastern margin of the Sea of Japan, where back-arc extension and strike-slip faulting contribute to crustal deformation. Depths consistently near 10 km align with the brittle-ductile transition zone typical of Honshu's continental crust.

References

SeismoSight internal swarm classification database.
Japan Meteorological Agency earthquake catalog.
USGS Earthquake Hazards Program regional tectonic summaries.