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Location:
59 km SSE of Shimoda, Japan
Period:
15 Aug 2000 15:02:58 - 16 Aug 2000 02:00:43 (10 hours 57 minutes)
Volcanoes in 100km radius:
Kozushima(10km), Miyakejima(29km), Niijima(29km), Toshima(43km), Mikurajima(46km), Izu-Oshima(67km), Izu-Tobu(85km), Kurose Hole(92km)
Earthquakes:
7
11 swarms found nearby.
2000
PS20000629.1(54.5km)
28 Jun
1 day 0 hours
10 earthquakes
PS20000703.1(15.6km)
2 Jul
7 hours
5 earthquakes
5 Jul
1 day 6 hours
27 earthquakes
11 Jul
2 days 5 hours
10 earthquakes
PS20000720.1(10.5km)
19 Jul
16 hours
6 earthquakes
23 Jul
1 day 5 hours
6 earthquakes
PS20000730.1(12.1km)
30 Jul
12 hours
5 earthquakes
2 Aug
21 hours
7 earthquakes
3 Aug
1 day 20 hours
45 earthquakes
2001
PS20010704.1(199.5km)
4 Jul
20 hours
12 earthquakes
2023
PS20230514.1(82.2km)
14 May
8 hours
8 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20000815.1: Earthquake Activity Southeast of Shimoda, Japan in August 2000

The Izu Peninsula region of Japan lies within a highly active tectonic zone at the boundary between the Philippine Sea Plate and the Eurasian Plate. Subduction along this margin drives frequent seismic events, including earthquake swarms, as well as volcanic activity associated with the Izu-Bonin arc. Shimoda, located on the southern tip of the peninsula in Shizuoka Prefecture, sits near offshore fault systems where such swarms commonly occur due to fluid migration and stress adjustments in the crust.

SeismoSight recorded Swarm PS20000815.1 beginning at 15:02 on 15 August 2000 and concluding at 02:00 on 16 August 2000. The sequence unfolded 59 km south-southeast of Shimoda and comprised seven earthquakes over approximately ten hours and fifty-seven minutes. All events occurred at a focal depth of 10 km.

The swarm initiated with a magnitude 5.2 earthquake at 15:02:58 on 15 August, followed rapidly by a magnitude 5.3 event at 15:55:26. Subsequent shocks included a magnitude 4.7 at 17:07:20, another 5.3 at 17:22:28, a 5.2 at 17:37:12, and a further 5.3 at 18:19:00. The final event, magnitude 5.1, occurred at 02:00:43 on 16 August. Magnitudes clustered between 4.7 and 5.3, characteristic of swarm behavior where no single mainshock dominates and energy releases in a concentrated temporal window.

This episode marked the first of nine documented swarms in the region since 1 January 2000. Such recurrent swarms reflect ongoing tectonic strain accumulation and release along the plate interface, consistent with the broader seismic history of the Izu Peninsula area.

Seismic swarms in this setting provide valuable data for monitoring crustal dynamics. The shallow 10 km depths indicate activity within the brittle upper crust, where small stress changes can trigger multiple similar-sized events. Continued observation helps refine models of subduction-related hazards in central Japan.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (general regional tectonics).
Japan Meteorological Agency seismic reports (Izu Peninsula geology).