Location:

Period:

19 Jul 2000 11:00:39 - 20 Jul 2000 03:10:25 (16 hours 9 minutes)

Volcanoes in 100km radius:

Kozushima(18km), Miyakejima(34km), Niijima(39km), Mikurajima(45km), Toshima(53km), Izu-Oshima(77km), Kurose Hole(87km), Izu-Tobu(94km)

Earthquakes:

11 swarms found nearby.

2000

PS20000629.1(61.4km)

28 Jun

1 day 0 hours

10 earthquakes

PS20000703.1(22.0km)

2 Jul

7 hours

5 earthquakes

S20000706.1(7.7km)

5 Jul

1 day 6 hours

27 earthquakes

PS20000712.1(7.2km)

11 Jul

2 days 5 hours

10 earthquakes

PS20000723.1(13.3km)

23 Jul

1 day 5 hours

6 earthquakes

PS20000730.1(13.3km)

30 Jul

12 hours

5 earthquakes

PS20000803.1(11.2km)

2 Aug

21 hours

7 earthquakes

S20000803.1(9.4km)

3 Aug

1 day 20 hours

45 earthquakes

PS20000815.1(10.5km)

15 Aug

10 hours

7 earthquakes

2001

PS20010704.1(191.4km)

4 Jul

20 hours

12 earthquakes

2023

PS20230514.1(73.7km)

14 May

8 hours

8 earthquakes

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Seismic Swarm PS20000720.1: Analysis of Activity South of Shimoda, Japan

The Izu Peninsula region south of Shimoda lies at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate. This tectonic setting produces frequent seismic swarms through a combination of subduction-driven stress accumulation, strike-slip faulting, and occasional magmatic intrusions along the northern Izu-Bonin arc. The peninsula itself formed through Quaternary volcanism and is cross-cut by active faults that accommodate oblique convergence at rates of several centimeters per year.

Seismic swarm PS20000720.1 was recorded 63 km south of Shimoda and represents the first of four such sequences documented in the region since 1 January 2000. The sequence began at 11:00 on 19 July 2000 and ended at 03:10 on 20 July 2000, spanning 16 hours and 9 minutes. All six events occurred at a focal depth of 10 km, consistent with shallow crustal faulting typical of the Izu Peninsula.

The earthquakes unfolded in two clusters. The initial three events on 19 July reached magnitudes 5.0, 5.2, and 5.4, with the largest occurring at 21:18. After a period of relative quiescence, four additional events took place within a nine-minute window on 20 July, including magnitudes 4.0, 5.1, and 5.3. The tight temporal spacing and uniform depth distribution indicate rapid stress release along a localized fault segment rather than a classic mainshock-aftershock sequence.

Earthquake swarms of this character are common in the Izu region and often reflect either aseismic slip on nearby faults or fluid migration linked to volcanic centers farther south. The 2000 swarm did not produce reported surface rupture or tsunami, consistent with its moderate magnitudes and offshore location. Historical records show that similar swarms have occurred repeatedly along the same tectonic corridor, underscoring the persistent seismic hazard.

Monitoring by the Japan Meteorological Agency continues to track microseismicity in the area, providing data that improve models of strain accumulation ahead of larger events. The recurrence of swarms since 2000 highlights the value of dense seismic networks for distinguishing swarm behavior from foreshock sequences that might precede damaging earthquakes.

References

Japan Meteorological Agency seismic catalog
USGS Earthquake Hazards Program regional tectonics summaries
Geological Survey of Japan, AIST, Izu Peninsula fault studies