Location:

Period:

11 Jul 2000 13:10:01 - 13 Jul 2000 18:55:52 (2 days 5 hours 45 minutes)

Volcanoes in 100km radius:

Kozushima(17km), Miyakejima(27km), Niijima(35km), Mikurajima(41km), Toshima(49km), Izu-Oshima(73km), Kurose Hole(86km), Izu-Tobu(92km)

Earthquakes:

11 swarms found nearby.

2000

PS20000629.1(54.2km)

28 Jun

1 day 0 hours

10 earthquakes

PS20000703.1(14.8km)

2 Jul

7 hours

5 earthquakes

S20000706.1(4.8km)

5 Jul

1 day 6 hours

27 earthquakes

PS20000720.1(7.2km)

19 Jul

16 hours

6 earthquakes

PS20000723.1(7.3km)

23 Jul

1 day 5 hours

6 earthquakes

PS20000730.1(7.1km)

30 Jul

12 hours

5 earthquakes

PS20000803.1(12.3km)

2 Aug

21 hours

7 earthquakes

S20000803.1(2.9km)

3 Aug

1 day 20 hours

45 earthquakes

PS20000815.1(6.6km)

15 Aug

10 hours

7 earthquakes

2001

PS20010704.1(193.0km)

4 Jul

20 hours

12 earthquakes

2023

PS20230514.1(75.8km)

14 May

8 hours

8 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20000712.1 Near Shimoda, Japan: Geological Context and Event Analysis

The Izu Peninsula region southeast of Shimoda lies within one of Japan's most tectonically active zones, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Nankai Trough and Suruga Trough systems. This convergence produces frequent shallow crustal earthquakes and occasional swarm sequences driven by both tectonic stress accumulation and magmatic fluid migration associated with nearby volcanic centers such as Mount Fuji and the Izu volcanic arc. Depths consistently near 10 km place events within the brittle upper crust, where brittle failure occurs readily under differential stress.

Swarm PS20000712.1 began at 13:10 on 11 July 2000 and concluded at 18:55 on 13 July 2000, spanning 53 hours and 45 minutes. During this interval, ten earthquakes were recorded at a location 50 km southeast of Shimoda. Magnitudes ranged from 4.2 to 5.3, with the strongest events reaching 5.3. All events shared a focal depth of 10 km, indicating a compact source volume typical of swarm activity rather than a single mainshock-aftershock sequence.

The temporal distribution shows clustering on 11 and 12 July, followed by a final event on 13 July. The initial trio of events on 11 July (magnitudes 5.3, 5.1, 5.2) established the swarm's intensity, while subsequent activity on 12 July included the weakest recorded event (magnitude 4.2) alongside several magnitude-5.0 shocks. This pattern reflects episodic stress release along distributed faults without a dominant rupture.

Since 1 January 2000, three swarms have been documented in the region, with PS20000712.1 representing the earliest. Such recurrent swarms underscore the Izu Peninsula's propensity for diffuse seismicity linked to ongoing plate-boundary deformation and hydrothermal processes.

Seismic swarms in this setting often precede or accompany minor volcanic unrest, although no surface deformation or eruptive activity was associated with this particular sequence. The uniform depth distribution suggests activation of a sub-horizontal fault network or fluid-driven fracturing at mid-crustal levels. Continued monitoring by the Japan Meteorological Agency and regional networks remains essential for distinguishing swarm behavior from foreshock sequences that could signal larger events.

References

Japan Meteorological Agency seismic catalogs (2000 event data)
Geological Survey of Japan, AIST: Tectonic framework of the Izu Peninsula
USGS Earthquake Hazards Program: Subduction zone seismicity of the Nankai Trough