Location:

Period:

5 May 2023 05:42:04 - 6 May 2023 14:54:30 (1 day 9 hours 12 minutes)

Volcanoes in 100km radius:

Niigata-Yakeyama(96km)

Earthquakes:

M 7.0+:

M 7.5; 2024 Noto Peninsula, Japan Earthquake; (1 Jan 2024) (7km from the swarm center)

2 swarms found nearby.

2024

PS20240101.1(28.8km)

1 Jan

18 hours

13 earthquakes

S20240101.3(13.6km)

1 Jan

2 days 3 hours

36 earthquakes

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Seismic Swarm S20230505.1: Near West Coast of Honshu, Japan

A seismic swarm designated S20230505.1 occurred near the west coast of Honshu, Japan, from 05:42 on 5 May 2023 to 14:54 on 6 May 2023. Over 33 hours and 12 minutes, 27 earthquakes were recorded, providing a concentrated sequence of activity in a tectonically active region.

The swarm initiated with a magnitude 6.2 event at a depth of 10 km. Subsequent events ranged from magnitude 3.5 to 5.6, with the majority occurring at depths of 9–10 km. Notable shocks included a magnitude 4.8 at 05:53:36, a magnitude 4.6 at 08:38:01, and a magnitude 5.6 at 12:58:03 on 5 May. The sequence concluded with a magnitude 4.1 event on 6 May. All events clustered tightly in both time and space, characteristic of swarm behavior where no single mainshock dominates.

This region lies along the western margin of Honshu, influenced by the complex interaction between the Eurasian, Philippine Sea, and Pacific plates. The Noto Peninsula area experiences ongoing crustal deformation due to subduction-related stresses and inland fault systems. Shallow depths around 10 km align with known active faults in the region, where brittle failure occurs in the upper crust.

Seismic swarms have been documented in this sector of Japan throughout modern monitoring history. They often reflect fluid migration or stress transfer along fault networks rather than a single large rupture. The May 2023 swarm fits this pattern, with rapid onset and gradual decay over roughly one day.

Notably, a magnitude 7.5 earthquake struck the Noto Peninsula on 1 January 2024, with its epicenter only 7 km from the swarm center. This proximity suggests possible long-term stress evolution in the same fault system.

The 2023 swarm underscores the value of dense seismic networks in capturing precursory or clustered activity. Continued monitoring remains essential given the region's history of damaging events driven by plate-boundary and intraplate tectonics.

References

Japan Meteorological Agency seismic catalogs
United States Geological Survey earthquake database
Geological Survey of Japan, AIST tectonic summaries