Seismic Swarm S20000706.1: A Detailed Examination of the July 2000 Event Off Shimoda, Japan

The seismic swarm designated S20000706.1 occurred approximately 61 km south-southeast of Shimoda on Japan’s Izu Peninsula. Registered between 20:10 on 5 July 2000 and 02:55 on 7 July 2000, the sequence lasted 30 hours and 45 minutes and comprised 27 earthquakes. All events were located at a focal depth of 10 km, with magnitudes ranging from 3.8 to 5.4.

The swarm initiated with a magnitude 4.8 event at 20:10 on 5 July. Activity intensified the following day, producing multiple shocks above magnitude 4.5, including a magnitude 5.2 at 06:27 and a magnitude 5.3 at 14:59. The sequence culminated in the largest event, magnitude 5.4, at 02:45 on 7 July. Subsequent smaller aftershocks declined rapidly, marking the end of the swarm within three hours.

The Izu Peninsula lies at the northern termination of the Izu-Bonin volcanic arc, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Sagami and Nankai troughs. This tectonic setting produces frequent moderate seismicity, often in swarm-like clusters associated with fluid migration or magmatic processes at shallow crustal depths. Historical records document recurrent earthquake swarms in the region, reflecting episodic strain release along secondary faults rather than a single major rupture.

Since 1 January 2000, two such swarms have been identified in the area, with S20000706.1 representing the earliest recorded cluster. The uniform 10 km depth across all events suggests activation of a discrete fault segment within the overriding plate, consistent with the regional geology of fractured volcanic and sedimentary units.

Analysis of the magnitude-time distribution shows a rapid onset followed by sustained activity over roughly 24 hours before tapering. The largest events occurred mid-sequence and at the close, illustrating typical swarm behavior in which no single mainshock dominates. Peak magnitudes remained below 6.0, indicating limited rupture dimensions and correspondingly low potential for widespread damage.

Seismic monitoring in this offshore sector benefits from dense networks operated by Japanese agencies, enabling precise relocation of events despite the marine setting. Continued observation of similar swarms contributes to refined models of stress transfer along the plate boundary and improves short-term forecasting capabilities for the Izu region.

References

Japan Meteorological Agency earthquake catalogue
SeismoSight internal swarm classification records