Seismic Swarm PS20000703.1: Analysis of Activity Near Shimoda, Japan

Seismic swarm PS20000703.1 occurred 31 km south-southeast of Shimoda on the Izu Peninsula, registering five earthquakes between 19:42 on 2 July 2000 and 03:36 on 3 July 2000. The sequence lasted seven hours and fifty-three minutes, with all events occurring at a focal depth of 10 km. Magnitudes ranged from 4.0 to 5.7, beginning with a 5.3 event, followed rapidly by a 5.7 shock, then three additional events of 5.2, 4.0, and 5.0.

This swarm represents the sole episode recorded in the region since 1 January 2000, underscoring its isolated character within the local seismic record. Such short-duration clusters typically reflect transient stress adjustments along minor faults rather than mainshock-aftershock sequences.

The Izu Peninsula lies at the northern termination of the Izu-Bonin volcanic arc, where the Philippine Sea Plate subducts beneath the Eurasian Plate. This tectonic configuration produces frequent shallow seismicity and occasional volcanic unrest. The peninsula’s geology features Quaternary volcanics, fault-bounded basins, and active strike-slip and reverse faults that accommodate oblique convergence. Historical records document destructive earthquakes in 1854 and 1923 that affected coastal communities near Shimoda, establishing the area’s long-term seismic hazard.

No additional swarms have been identified in the same source zone through the present, consistent with the single-episode classification since 2000. The 2000 events therefore provide a useful reference for understanding episodic, low-magnitude clustering in this subduction-related setting.

References

• Japan Meteorological Agency earthquake catalog (2000 events).
• Geological Survey of Japan, AIST: Tectonic framework of the Izu Peninsula.
• SeismoSight internal swarm classification PS20000703.1.