Location:

Period:

2 Aug 2000 21:42:27 - 3 Aug 2000 19:23:38 (21 hours 41 minutes)

Volcanoes in 100km radius:

Kozushima(8km), Niijima(30km), Miyakejima(37km), Toshima(43km), Mikurajima(53km), Izu-Oshima(68km), Izu-Tobu(83km), Kurose Hole(97km)

Earthquakes:

10 swarms found nearby.

2000

PS20000629.1(62.0km)

28 Jun

1 day 0 hours

10 earthquakes

PS20000703.1(23.5km)

2 Jul

7 hours

5 earthquakes

S20000706.1(7.7km)

5 Jul

1 day 6 hours

27 earthquakes

PS20000712.1(12.3km)

11 Jul

2 days 5 hours

10 earthquakes

PS20000720.1(11.2km)

19 Jul

16 hours

6 earthquakes

PS20000723.1(11.6km)

23 Jul

1 day 5 hours

6 earthquakes

PS20000730.1(19.1km)

30 Jul

12 hours

5 earthquakes

S20000803.1(15.1km)

3 Aug

1 day 20 hours

45 earthquakes

PS20000815.1(7.9km)

15 Aug

10 hours

7 earthquakes

2023

PS20230514.1(84.9km)

14 May

8 hours

8 earthquakes

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Earthquake Swarm PS20000803.1: Seismic Activity Near Shimoda, Japan

An earthquake swarm designated PS20000803.1 was recorded 53 km south-southeast of Shimoda on Japan’s Izu Peninsula. The sequence began at 21:42 on 2 August 2000 and concluded at 19:23 on 3 August 2000, spanning 21 hours and 41 minutes. Seven events were registered during this interval, with magnitudes ranging from 4.6 to 5.4 and focal depths consistently at 10 km.

The swarm comprised the following events: a magnitude 5.3 earthquake at 21:42 on 2 August; a second magnitude 5.3 event at 07:55 on 3 August; a magnitude 5.1 shock at 11:27; a magnitude 4.6 event at 12:06; the largest shock of magnitude 5.4 at 12:13; another magnitude 5.3 event at 13:18; and a final magnitude 5.1 earthquake at 19:23. All events clustered tightly in both time and space, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

The Izu Peninsula lies at the northern tip of the Philippine Sea Plate, where oblique subduction beneath the Eurasian Plate drives frequent seismic and volcanic activity. The region forms part of the Izu-Bonin arc system, with active faults and volcanic centers contributing to elevated seismicity. Historical records document recurrent earthquake swarms in this offshore area, often linked to fluid migration or magma movement at shallow depths. Since 1 January 2000, seven such swarms have occurred in the vicinity, with the 2000 sequence representing the earliest in the modern instrumental catalog for this specific locale.

Swarm activity in the Izu region typically features closely spaced events of moderate magnitude occurring over hours to days. Depths around 10 km align with the brittle-ductile transition zone influenced by the local tectonic regime. These patterns provide insight into stress transfer along the plate interface and subsidiary faults, aiding regional hazard assessment without indicating immediate volcanic unrest.

Geological studies of the Izu Peninsula emphasize its complex history of arc volcanism and rapid uplift, with ongoing convergence rates of approximately 4–5 cm per year. The 2000 swarm fits within this framework of persistent microplate interactions that have shaped the peninsula’s landscape over millions of years.

References

Japan Meteorological Agency seismic bulletins (2000)
USGS Earthquake Catalog and tectonic summaries for the Izu-Bonin region
Geological Survey of Japan reports on Izu Peninsula seismotectonics