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Location:
62 km SW of Yonakuni, Japan
Period:
8 Nov 2004 15:35:07 - 8 Nov 2004 19:38:16 (4 hours 3 minutes)
Volcanoes in 100km radius:
Kueishantao(68km)
Earthquakes:
5
M 7.0+:
M 7.1; 67 km ENE of Hualien City, Taiwan; (31 Mar 2002) (47km from the swarm center)
11 swarms found nearby.
2002
PS20021024.1(138.6km)
23 Oct
10 hours
11 earthquakes
2013
S20130417.2(68.2km)
17 Apr
1 day 3 hours
41 earthquakes
2015
PS20150420.1(58.9km)
20 Apr
10 hours
5 earthquakes
2018
PS20180206.1(100.0km)
6 Feb
23 hours
7 earthquakes
2020
S20200615.2(83.9km)
14 Jun
1 day 20 hours
30 earthquakes
2022
PS20220322.1(162.0km)
22 Mar
2 hours
7 earthquakes
PS20220918.1(187.4km)
17 Sep
1 day 12 hours
11 earthquakes
2024
PS20240403.1(90.5km)
2 Apr
2 hours
21 earthquakes
S20240403.1(80.2km)
3 Apr
5 days 4 hours
102 earthquakes
PS20240422.1(123.2km)
22 Apr
17 hours
27 earthquakes
S20240422.1(113.3km)
22 Apr
2 days 23 hours
84 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20041108.1: Analysis of Activity Southwest of Yonakuni, Japan

On 8 November 2004, a seismic swarm designated PS20041108.1 was recorded 62 km southwest of Yonakuni, Japan. The sequence began at 15:35 UTC and concluded at 19:38 UTC, encompassing five earthquakes within a span of four hours and three minutes. Magnitudes ranged from 5.1 to 6.3, with focal depths between 28 km and 60 km. The largest events reached magnitude 6.3 at 15:55 UTC.

The swarm unfolded in two distinct clusters. The initial phase featured a magnitude 5.1 event at 15:35, followed immediately by paired magnitude 6.3 shocks at depths of 29 km and 40 km. Activity resumed in the evening with a magnitude 5.2 event at 19:38, succeeded seconds later by another magnitude 5.2 shock at 60 km depth. This tight temporal grouping and moderate depth range are characteristic of swarm behavior in subduction-influenced settings.

Yonakuni lies at the western end of the Ryukyu Islands arc, where the Philippine Sea Plate subducts beneath the Eurasian Plate along the Ryukyu Trench. This convergent margin produces frequent seismicity, with intermediate-depth events common due to slab dehydration and stress accumulation. The swarm location aligns with the forearc region, influenced by both the trench and nearby transform faults associated with the opening of the Okinawa Trough.

Historical records since 2000 indicate limited swarm occurrences in the immediate vicinity. One prior swarm took place in 2002. A notable regional event was the magnitude 7.1 earthquake on 31 March 2002, located 67 km east-northeast of Hualien City, Taiwan, approximately 47 km from the 2004 swarm center. Such activity underscores the persistent tectonic loading across the plate boundary.

Insights from the 2004 swarm highlight rapid energy release without a clear mainshock-aftershock pattern. The dual magnitude 6.3 events suggest possible triggering along adjacent fault segments within the subducting slab. Depths clustering around 30–40 km for the strongest shocks point to processes near the plate interface.

References

SeismoSight internal swarm classification PS20041108.1
USGS Earthquake Catalog (regional tectonics)
Global CMT Project (subduction zone parameters)