Seismic Swarm S20000803.1 Offshore Shimoda, Japan

An earthquake swarm designated S20000803.1 occurred 79 km south-southeast of Shimoda on the Izu Peninsula. The sequence began at 07:20 on 3 August 2000 and concluded at 03:55 on 5 August 2000, spanning 44 hours and 35 minutes. During this interval, 45 earthquakes were recorded, with magnitudes ranging from 3.6 to 4.9 and hypocentral depths consistently near 10 km.

The swarm initiated with two events above magnitude 4.0 within the first two minutes. Activity intensified through the morning of 3 August, producing multiple shocks of magnitude 4.5–4.8. The largest event, magnitude 4.9, occurred at 09:04. Subsequent hours featured repeated clusters of magnitude 4.2–4.7 earthquakes, with the final event of the sequence registering magnitude 3.8 on 5 August. All events shared similar focal depths, indicating a compact source volume.

This swarm represents the first of eight documented swarms in the region since 1 January 2000. Such sequences are characteristic of the tectonic environment surrounding the Izu Peninsula, where the Philippine Sea Plate subducts beneath the Eurasian Plate. The resulting stress regime produces both isolated large earthquakes and episodic swarms along offshore fault systems.

The Izu Peninsula lies at the northern termination of the Izu-Bonin volcanic arc. Ongoing arc-continent collision and oblique subduction generate shallow crustal seismicity, often concentrated between 5 and 15 km depth. Historical records show recurrent swarm activity in this offshore sector, frequently linked to fluid migration or aseismic slip transients rather than magmatic intrusion.

Seismic swarms in this setting typically last from hours to several days and rarely culminate in a single dominant mainshock. Instead, energy is released through numerous events of comparable size, consistent with the magnitude distribution observed in S20000803.1. Monitoring by the Japan Meteorological Agency continues to track similar patterns, providing data for refined hazard assessment along the Suruga Trough and adjacent plate boundary segments.

References

Japan Meteorological Agency earthquake catalog
United States Geological Survey earthquake archive
SeismoSight internal swarm classification database